This is apl.info, produced by makeinfo version 5.2 from apl.texi.

This is the manual for GNU APL, an implementation of ISO standard 13751
"Programming Language APL, Extended"

   Copyright © 2013 Free Software Foundation, Inc.
INFO-DIR-SECTION GNU programming tools
START-INFO-DIR-ENTRY
* APL: (apl).                  the programming language APL - ISO 13751.

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GNU APL
*******

This manual briefly documents GNU APL, an interpreter for APL as
described in ISO standard 13751, aka.  "Programming Language APL,
Extended".

   This manual for GNU APL can be distributed under the terms of the GNU
Free Documentation License, *Note Chapter 6::.

   This manual does NOT describe APL itself; there exist a number of
good books and texts about APL. We would like to refer the reader to the
following:

   ISO Standard for APL:
www.math.uwaterloo.ca/~ljdickey/apl-rep/docs/is13751.pdf

   More pointers to APL resources on the Web:
ftp://rtfm.mit.edu/pub/usenet-by-group/comp.lang.apl/APL_language_FAQ

* Menu:

* Chapter 1::    Installing and Starting GNU APL
* Chapter 2::    Design Decisions
* Chapter 3::    Non-standard Features
* Chapter 4::    Limitations and Missing Features
* Chapter 5::    Internationalization
* Chapter 6::    Libraries Contributed by GNU APL Users
* Chapter 7::    Licenses for GNU APL and this Manual


File: apl.info,  Node: Chapter 1,  Next: Chapter 2,  Prev: Top,  Up: Top

1 Installing and Starting GNU APL
*********************************

1.1 Installing APL
==================

APL is built and installed like this:

   • download the tar file containing GNU APL, from
     <ftp://ftp.gnu.org/gnu/apl> or from one of its mirrors (see
     <http://www.gnu.org/prep/ftp.html>), and untar the file.

   • read (and follow) the instructions given in file README in the
     unpacked directory

   For the experienced, but impatient reader: it is the usual sequence

   • ./configure

   • make

   • make install

   On success, an executable file called ’apl’ will have been produced
in the sub-directory ’src’.

   File README contains further instructions about the installation GNU
APL.

1.2 Starting APL
================

Last things first: before explaining how to start APL, it is important
to remember how to stop (i.e.  exit) it.  Neither ^C nor ^D will stop
APL - they serve other purposes.  Instead, you leave APL with the
command )OFF (on a separate line) like this:

   )OFF

   If APL is computing a function (and possibly caught in an endless
loop), then you may have to press ^C (called ATTENTION in APL) in order
to return to APL’s command mode and then give the )OFF command.

   Having that said, APL is started like every other program - entering
its name and optional command line parameters, for example:

   $ apl

   or:

   $ apl —id 1001

   GNU APL is script-able; a text file whose first line looks like this
(assuming the APL interpreter binary is called ’apl’ and is located in
the current directory):

   #!  apl

   or (if the APL interpreter binary is not in the current directory but
in /mypath/apl):

   #!  /mypath/apl

   The path to the ’apl’ binary can be missing (like in the first
example), relative, or absolute (second example).  The exact details of
how the first line of a GNU APL script shall look like varies slightly
among operating systems.  Please consult the info (or man) pages for
’path_resolution’ and/or ’execve’ on your operating system.  After the
the name of the binary, optional GNU APL command line options, usually
–script, can be provided.

   The text file must, of course, have execute permission, and should be
ASCII or, more likely, UTF-8 encoded.  The APL characters in the script
shall be those defined in the Unicode character set (most of them in the
U+2200 - U+23FF range).

1.3 Command Line Options
========================

GNU APL understands the following command line options:

   • ‘--cfg’

     show ./configure options that were used to configure GNU APL, and
     exit.

   • ‘--[no]Color’

     start with ]COLOR ON [OFF].

   • ‘-d’

     run the APL interpreter (or APL script) in the background (i.e.  as
     a daemon).  For this to work you need to provide some input to the
     background process, e.g.  via the -f option.

   • ‘--emacs’

     run in (old) Emacs mode.

   • ‘--emacs_arg arg’

     run in (new) Emacs mode with argument arg.

   • ‘-f file’

     read input from \fIfile\fP rather than from the keyboard.  When the
     end of the file is reached, input is switched back to the keyboard.
     If you want to terminate the APL interpreter after executing the
     file, then use )OFF as last line in the file.

   • ‘--gpl’

     show GNU APL license (GPL) and exit.

   • ‘-L wsname’

     )LOAD wsname on start-up.

   • ‘--LX expr’

     execute expr first.  This can be used, for example, to load some
     library or workspace at start-up.

   • ‘-h’, ‘--help’

     print all command line options with a brief hint what they do.

   • ‘--id proc’

     use processor ID proc for this interpreter.  If no ID is provided,
     then the first unused ID > 1000 is taken by this interpreter and
     the ID becomes used as long as the interpreter runs.  Processor IDs
     are used by shared variables to identify share partners.

   • ‘-l num’

     turn logging facility num ON (if dynamic logging is configured).
     The logging facility for start-up messages can be turned on even
     when dynamic logging is not configured.

   • ‘--echoCIN’ copy the input line (after editing) to stdout.  For
     creating session logs.

   • ‘--noCIN’

     do not echo stdin to stdout.  Almost a must for scripting (unless
     you troubleshoot a script).

   • ‘--noCONT’

     do not load a SETUP or CONTINUE workspace on start-up.

   • ‘--PW COLS’

     set the initial value of ⎕PW to COLS (min.  30, max.  10000)

   • ‘--[no]SV’

     do [not] start APserver (a shared variable server) on start-up.
     This disables communication with other workspaces or auxiliary
     processors through shared variables.

   • ‘-p N’

     use profile number N in preferences files.

   • ‘--par pproc’

     use processor parent ID pproc (default: no parent ID).

   • ‘--rawCIN’

     do not emit ESC sequences.  Normally ESC sequences are emitted for
     colored output and during line editing.  In scripts, however, ESC
     sequences usually not wanted and can be turned off with this
     option.

   • ‘-s’, ‘--script’

     this option is an abbreviation for: ‘--silent’ ‘--noCIN’ ‘--noCONT’
     ‘-f -’ which is a typical combination of options for APL scripts.

   • ‘-q’, ‘--silent’

     suppress printing of the GNU APL welcome message.  Useful for
     scripts.

   • ‘--safe’

     disable shared variables and native functions

   • ‘--show_bin_dir’

     display the binary directory (where, according to ./configure, the
     programs apl, APserver, AP100, and AP210 are supposed to be
     installed.  Then exit.

   • ‘--show_doc_dir’

     display the directory where, according to ./configure,
     documentation files for GNU APL are installed.  Then exit.

   • ‘--show_etc_dir’

     display the system configuration directory where, according to
     ./configure, the preferences file for GNU APL is installed.  Then
     exit.

   • ‘--show_lib_dir’

     display the library directory where, according to ./configure,
     shared library files and the workspaces shipped with GNU APL are
     installed.  Then exit.

   • ‘--show_src_dir’

     display the source directory where, according to ./configure, GNU
     APL was compiled.  Then exit.  This can be used, for example, by
     native functions\ that are built outside the GNU APL source tree to
     find GNU APL header files that are needed to compile the native
     function.

   • ‘--show_all_dirs’

     display all the directories above.  Then exit.

   • ‘-T testcases ...’ run testcases.  Testcases are text files that
     contain both input to the APL interpreter and the expected response
     from the interpreter.  The output from the interpreter is compared
     with the expected output in the testcase file(s) and differences
     are marked.  In addition a summary file is created that tells
     whether or not each of the testcases was successful.

   • 
     ‘--TM mode’

     test mode.  This option how the interpreter shall behave when
     running a number of testcases (as specified with the ‘-T’ option)

     ‘--TM 0’ (default) run all testcases and exit after the last
     testcase.

     ‘--TM 1’ like ‘--TM 0’ if no error was detected.  However, if one
     of the testcases has failed, then the interpreter does not exit so
     that the user can investigate the state of APL (SI, variable
     values, etc.).

     ‘--TM 2’ like ‘--TM 1’, but stay in the interpreter even if all
     testcases have passed.  This can be useful in order to quickly
     bring the interpreter into a specific state and continue manual
     troubleshooting from that state.

     ‘--TM 3’ like ‘--TM 1’, but stop testcase execution after the first
     failed testcase (i.e.  do not exit).

     ‘--TM 4’ like ‘--TM 3’, but exit after the first failed testcase.
     The is useful for automatic regression tests, where no errors are
     expected.

   • ‘--TR’

     executes test case files in random order.

   • ‘--TS’

     Normally, when the interpreter is run with the ‘-T’ option, then an
     existing summary.log file is overridden without notice.  This
     option causes new test results to be appended to a possibly
     existing summary.log instead of overriding it.

   • ‘-v’, ‘--version’,

     show version information and exit.

   • ‘-w milli’

     wait milli milliseconds at start-up.  Useful to give other programs
     that are started together with this interpreter time to initialize
     themselves.

   • ‘--’

     end of command line options for the interpreter.  GNU APL provides
     the system variable ⎕ARG that returns all command line options with
     which the GNU APL interpreter was started (similar to variable argv
     in main(int argv, char * argv[]) in C/C++).  Option —- can be used
     to separate command line options for the APL interpreter from
     command line options understood by APL applications.

     Thus,

     ‘(⎕ARG ⍳ ⊂'--') ↑ ⎕ARG’ returns the options for the APL
     interpreter, while

     ‘(⎕ARG ⍳ ⊂'--') ↓ ⎕ARG’ returns the options for the APL
     application.

     All command line options after ‘--’ are ignored by the interpreter
     (except making them available in ⎕ARG).

1.4 Config File for GNU APL
===========================

The default values for some of the command line options discussed in the
previous section can be set in a configuration file for GNU APL. The
name of the configuration file is ’preferences’ and it should live in
one of the following directories:

   • in the sub-directory gnu-apl.d of the system configuration
     directory, or

   • in the sub-directory .gnu-apl of the user’s home directory (as per
     $HOME).

   The system configuration directory is usually /etc or /usr/local/etc
and is configurable via ‘./configure --sysconfdir’.  An empty (i.e.  all
settings commented out) preferences file is being installed in the
system configuration directory when GNU APL is installed.  You can edit
it or use it as a template to see which options can be controlled.

   If file ’preferences’ exists in both directories, then the settings
in $HOME/.gnu-apl/preferences override settings in, for example,
/etc/gnu-apl.d/preferences.

   Command line options in turn override settings in ’preferences’
files.

1.5 File Names and Paths
========================

The GNU APL interpreter is a binary file named apl.  It is usually
installed in directory /usr/bin/ or in /usr/local/bin/.  The location
where apl is installed can be changed via ./configure options (see file
INSTALL).

   GNU APL understands 4 file types:

  1. APL workspaces that can be manipulated with the )LOAD, )SAVE,
     )COPY, and )DROP commands.  APL workspaces are XML files and must
     have a file extension of .xml in order to be accepted by GNU APL.
     APL workspaces can only be exchanged between machines that all run
     GNU APL.

  2. APL exchange files can be manipulated with the )IN and )OUT
     commands.  APL exchange files are text files in ⎕TF format defined
     by IBM (basically APL expressions that create variables or
     functions) and must have a file extension of .atf in order to be
     accepted by GNU APL. APL exchange files can be exchanged between
     machines running APL interpreters from different vendors.  The ⎕TF
     format can be easily emulated on machines not providing compatible
     )IN and )OUT commands.

  3. APL scripts consist of APL commands and APL expressions (including
     function definition via ∇) like they would be entered by the user.
     APL scripts should have a file extension of .apl but other
     extensions are also accepted by GNU APL. APL scripts are, for
     example, the files expected for the -f command line option.  A
     workspace can be written in this format with the )DUMP command.

  4. APL testcase files are similar to APL scripts, but in addition to
     the APL commands and expression they also contain the expected
     output from from the commands APL testcase files normally have have
     a file extension of .tc for normal (functional) testcases and .pt
     for performance testcases.  APL testcase files are, for example,
     the files expected for the -T command line option.

   The following APL commands are related to file names:

   )LOAD [lib] name[.xml]

   )SAVE [lib] [name[.xml]]

   )COPY [lib] [name[.xml]]

   )PCOPY [lib] [name[.xml]]

   )DROP [lib] [name[.xml]]

   )IN [lib] name[.atf]

   )PIN [lib] name[.atf]

   )OUT [lib] name[.atf]

   )DUMP [lib] [name[.apl]]

   )LIB [lib]

   )LIBS [new-lib-root]

   )WSID [name]

   The rules how file names are constructed from the argument(s) of an
APL command are:

  1. command arguments shown in brackets are optional.

  2. lib is a number from 0 to 9.  If lib is not present then 0 is taken
     as default.

  3. if the name is optional and missing then the workspace ID (the name
     set with )WSID) is used.

  4. if the file extension (i.e.  .xml or .atf) is missing then it is
     appended automatically to name.

   If the name starts with ’/’ then it is taken as an absolute path to
the file (an absolute file name) and no further computation is done with
the name.

   Otherwise name is a relative path which is relative to some directory
library-root and a sub-directory of library-root that is determined by
the lib number.  The library numbers 0-9 correspond to the following
directories:

   0: library-root/workspaces/

   1: library-root/wslib1/

   2: library-root/wslib2/

   ...

   9: library-root/wslib9/

   The command )LIBS without arguments shows the mapping between library
numbers and paths.  The command )LIBS with an argument sets a new
lib-root.

   The command )LIB [lib] shows the files in library (i.e.  directory)
lib.

   The directory library-root is computed as follows when the
interpreter starts:

   If an environment variable APL_LIB_ROOT is defined, then its value is
used as library-root.  Otherwise the path from the current directory
(".")  up to the root directory ("/") is searched until a directory
containing two files ’workspaces’ and ’wslib1’ is found.  Normally
’workspaces’ and ’wslib1’ are directories, but for the computation of
library-root files suffice.

   If such a directory is found, then it is used as library-root;
otherwise the current directory (i.e.  ".")  is used and converted to an
absolute path.

   For example, if library-root is "."  then the command

   )LOAD 2 test

   will try to load the workspace file

   ./wslib2/test.xml

   Using a library root implies that all 10 library directories are
contained in the same directory.  This is good enough for single-user
environments but is often not adequate for multi-user environments where
some directories are not writable by users and different users have
different home directories.

   For that reason the above library root scheme can be overridden by
the GNU APL configuration files (preferences).  In these files you can
uncomment any of the LIBREF-0 to LIBREF-9 settings (which correspond to
library numbers 0 to 9) and provide your own paths.  The library numbers
NOT overridden in a ’preferences’ file still follow the library root
scheme.


File: apl.info,  Node: Chapter 2,  Next: Chapter 3,  Prev: Chapter 1,  Up: Top

2 Design Decisions
******************

The ISO standard leaves a few things unspecified and GNU APL may behave
differently than other APL interpreters.

2.1 APL symbols that can be functions or operators
==================================================

There are 4 APL symbols / ⌿ \ and ⍀ that can be functions or operators,
depending on their context (i.e.  depending on the APL tokens left of
them).  Neither the ISO standard nor the otherwise excellent IBM APL2
language reference manual explain the rules that decide whether, for
example /, should mean ’function compress’ or ’operator reduce’.  In
simple cases the meaning of / is obvious: +/B means ’operator reduce’
while 1/B means ’function compress’.

   The following example may explain the problem:

     ÷  /  \  2      ⍝ case (i)
2
   (÷  /) \  2       ⍝ case (ii)
2
   ÷ (/  \) 2        ⍝ case (iii)
0.5

   In (i) and (ii) / is an operator thanks to ÷ left of it while in
(iii) / is a function bound to operator \.

   Both (ii) and (iii) are valid interpretations if (i).  The ISO
standard defined the result after the decision if / should be treated as
function or as operator was made but says nothing about how to decide
that.

   The APL2 language reference gives additional information about such
aspects (binding strength between token) but the ambiguity related to /
⌿ \ and ⍀ is not resolved by these rules.

   The first decision to make is at which point in time the role of /
and friends shall be decided.  The options are:

   (a) when the statement is tokenized and parsed (call that moment ’⎕FX
time’ similar to compile-time for compiler languages), or

   (b) when a statement is executed (call that ’run-time’), or

   (c) theoretically a combination of (a) and (b)

   The advantage of (a) is less run-time overhead which is important
because / is probably the most frequently used APL operator.  As will be
explained below there are cases where (a) fails.  The advantage of (b),
at first glance, seems to be that it always gives the correct (more
precisely: the expected result because correct would require a valid
specification).  A closer look reveals, however, that this is only true
for the most simple cases but not in general.

   To give you an idea where (b) becomes tricky is this (there are more
of that kind):

   A[FOO]/B ⍝ where FOO could define A via ⎕FX or do A←

   It is still possible to resolve the last example at run time, but the
parser would become far more complex to do that.  The same holds for
(c).

   For these reasons the decision was made in favor of (a) with a
syntactic tweak to handle the (single) case where (a) fails.  In
principle the symbol left of, for instance, / decides if / should be a
function or an operators.  The matter can become more complex if index
brackets or parentheses are involved.  The algorithm used in GNU APL for
classifying / and friend is (simplified) this:

   1.  / ⌿ \ and ⍀ are tokenized as operators

   2.  Scan the tokenized statement from left to right to find / ⌿ \ or
⍀

   3.  If a / ⌿ \ or ⍀ is found, go backwards until a constant, or a
function, or a user-defined name is found: 3a.  if a constant, a
function, or a user-defined name is found then →4 3b.  if ) is seen then
skip over it, That is, a parenthesized expression is entered.  3c.  if ]
is seen then skip until the corresponding [ is found, That is an index
list or axis is skipped over.  3d.  otherwise: SYNTAX ERROR

   4.  decide the role of / \ ⌿ or ⍀ based on the kind of token that was
found in 3a: 4a.  function: leave / as operator as initially tokenized
4b.  constant: change / from operator to function (compress) 4c.  see
below

   This works fine except in case 4c.  which cannot be resolved at ⎕FX
time.  For this case we introduce the following convention:

   A user-defined name is assumed to be a function (and hence / is an
operator) unless the name (and nothing else) is enclosed in parentheses.
For example:

       A  / B    ⍝ A is function, therefore / is operator

      (A) / B    ⍝ A is value, therefore / is function

   This rule is moderately compatible with ISO and APL2; in APL2 (A)
would be redundant parentheses so in that case there would be a small
incompatibility between GNU APL and IBM APL2.


File: apl.info,  Node: Chapter 3,  Next: Chapter 4,  Prev: Chapter 2,  Up: Top

3 Non-standard Features
***********************

* Menu:

* Section 3.1::    APL Scripting
* Section 3.2::    ⎕SYL - System limits
* Section 3.3::    Dyadic ⎕CR
* Section 3.4::    ⎕FIO: File I/O Functions
* Section 3.5::    Debug Commands
* Section 3.6::    Colored Output
* Section 3.7::    Logging Facilities
* Section 3.8::    Complex Numbers
* Section 3.9::    Native Function
* Section 3.10::   DUMP Command
* Section 3.11::   User-defined Commands
* Section 3.12::   Direct Functions (Lambdas)
* Section 3.13::   Axis argument in defined functions
* Section 3.14::   ⎕SI - State Indicator
* Section 3.15::   ⎕DLX: Knuth’s Dancing Links Algorithm
* Section 3.16::   History and TAB completion
* Section 3.17::   ⎕SQL - SQL Database Interface
* Section 3.18::   Comparison Rules

There are a few possibly useful features in GNU APL:


File: apl.info,  Node: Section 3.1,  Next: Section 3.2,  Up: Chapter 3

3.1 APL Scripting
=================

As already mentioned, it is possible to write APL scripts.  Like for
other script languages, the script starts with ‘#!’, followed by the
absolute or relative path to the interpreter, followed by command line
arguments passed on to the interpreter.  In our case this could be, for
example:

    #! /usr/local/bin/apl --id 1010

   There are essentially two ways to run an APL script: redirecting the
script file to stdin of the interpreter or making the script executable
and indicate apl as the script interpreter (followed by some command
line arguments for apl).

3.1.1 redirect the script file the stdin of the GNU APL interpreter
-------------------------------------------------------------------

A file, say SCRIPT.apl, can be redirected to apl by redirection of the
shell:

    apl < SCRIPT.apl

   Alternatively, the command line option -f of apl can be used:

    apl -f SCRIPT.apl

   Both ways of redirecting the file are almost identical; in the first
case redirection was performed by the shell running apl, while in the
second case the redirecting of the file was performed by apl itself.

3.1.2 Make the script file executable
-------------------------------------

Alternatively the script can be made executable, mentioning apl as the
interpreter for the script (see also ‘'man 2 execve'’).  For this to
work, the first line of the script must have a special format

    #! /usr/local/bin/apl --script

   ‘--script’ prevents: printing of a welcome banner, echoing of stdin
to stdout, automatic loading of a )CONTINUE workspaces, and prevents
starting of a thread for shared variable communication.

   If the path to the interpreter is relative (which might be useful if
you do not know in advance where the APL interpreter will be installed)
then it must be in $PATH of the shell that runs the script).

3.1.3 How command line arguments are handled
--------------------------------------------

As we have seen, GNU APL can be started directly (by entering the name
of the GNU APL binary on the command line of a shell, or indirectly by
entering the name of a script that provides the name of the GNU APL
binary on the first line of the script.  In the latter case, the shell
uses function execve() to start APL, and one can provide additional
arguments that are passed on to APL. We briefly discuss both cases:

   0.  General Remark

   There is a convention in GNU APL that command line options (and
possibly option arguments) that occur left of ’—-’ are options directed
to (and understood by) the APL interpreter while options right of ’—-’
are passed to the APL application (and their exact format is defined by
the application).  The command line options that were used to start APL
can be retrieved with ⎕ARG.

   Example 1: APL started directly

   We start APL with some command line options and display them with
⎕ARG. The command line options for the interpreter are –silent, –id, and
1010, while the options app1 and app2 are for the APL application:

$ apl --silent -l 37 -- app1 app2
    ⊃⎕ARG
apl
--silent
-l
37
--
app1
app2

   Example 2: APL started indirectly

   Starting APL indirectly means that there is a script (a text file
with execute permission) that contains APL code and has a special first
line as described above.  There are now two sets of command line
options: those passed to the script and those on the first line of the
scripts.  When the script is started, possibly with command line
arguments, then the command line arguments entered by the user are
combined with the arguments on the first line of the script in a way
that is described in ’man 2 execve’.

   Assume the script file SCRIPT.apl contains this (the file is shipped
with GNU APL):

#! /usr/local/bin/apl --id 1010 --script

      ⊃⎕ARG      ⍝ show command line options
      )OFF      ⍝ leave the interpreter


   Assume further that the script is started like this:

$ ./SCRIPT.apl sarg1 sarg2

   Then the following output, which shows the order of command line
options as seen by ⎕ARG, is produced:

   ⊃⎕ARG
/usr/local/bin/apl
--script
./SCRIPT.apl
sarg1
sarg2

   This tells us that the different names are constructed by execve() in
the following order:

   1.  Interpreter name (the first name in the script file, here:
/usr/local/bin/apl)

   2.  Optional Interpreter arguments (here: —-script)

   3.  Script filename (here: ./SCRIPT.apl)

   4.  Script arguments (here: sarg1 sarg2)

   CAUTION: as discussed in ’man 2 execve’ the interpretation of the
optional interpreter arguments (2.  above) is system specific and
possibly not portable.  For that reason the optional interpreter
argument should not be missing (you can use —- to provide at least one
argument) and also not more than one argument.  On GNU/Linux systems the
following first lines of the script are understood:

/usr/local/bin/apl
/usr/local/bin/apl --
/usr/local/bin/apl -s
/usr/local/bin/apl --script
/usr/local/bin/apl -s --
/usr/local/bin/apl --script --

   The recommended form is: /usr/local/bin/apl —-script

   Note that ’—-’ and options that follow it ARE NOT shown in ⎕ARG if
they are given on the first line of the script, while ’—-’ and options
that follow on the script command line ARE shown.  There is no point in
passing application options in the script file via the command line to
apl; instead you can use them directly in the script.

   Using ’—-’ as last argument on the first line of the script file
prevents any of the options given to the script to be interpreted as APL
options; all such options are passed to the application via ⎕ARG.

3.1.4 Helpful features for scripting
------------------------------------

In order to be really useful for scripting, three more features have
been added:

   The system variable ⎕ARG contains all command line arguments passed
to the interpreter.  In the initial example above this would be a nested
3-element vector:

    /usr/local/bin/apl --id 1010

   A feature particularly useful for CGI scripting (where some
information is passed to the APL script via environment variables) is
the monadic system function ⎕ENV B. ⎕ENV B returns all environment
variables whose name starts with B. If ⍴B is 0, like:

    ⎕ENV ''

   then all environment variables are returned.  The result of ⎕ENV is a
N-by-2 matrix.  The first column is the name of the environment
variable.  The second column is the value of the variable.  The number N
of environment variables returned obviously depends on the number of
environment variables whose name begins with the prefix given in B. B is
case sensitive, but by convention the names of environment are
uppercase.

   If you need to create a longer text, like the body of a web page,
then you would normally construct it like this:

BODY ← ⊂ 'First line'
BODY ← BODY , ⊂ 'Second line'
BODY ← BODY , ⊂ 'Third line'
   ...

   That is not very handy for longer texts.  Instead you can write the
following in an APL script:

BODY←⎕INP 'END-OF-⎕INP'
First line
Second line
Third line
   ...
END-OF-⎕INP

   This works like "Here documents" in bash.  The right argument of ⎕INP
is the end of text marker, ⎕INP also has a dyadic form where the left
argument specifies an escape sequence from text to apl and back to the
text, similar to PHP scripts:

   ’<?apl’ ’?>’ ⎕INP ’END-OF-⎕INP’ First line Time is now: <?apl ⍕⎕TS ?>
Third line ...  END-OF-⎕INP

   These features together allow you to write Web pages in APL!

3.1.5 Script Example
--------------------

Note that the two different ways of running an APL script have an impact
on how ⎕ARG looks like.  If stdin is redirected then there is only one
(possibly empty) set of command line options.  Otherwise there are two
set of command line options: command line options for the apl
interpreter and command line options for the script.

   Consider the following simple script called SCRIPT.apl in directory
workspaces:

#! /usr/local/bin/apl --script

⊃⎕ARG    ⍝ show command line options
)OFF    ⍝ leave the interpreter

   If SCRIPT.apl is redirected to stdin of the APL interpreter:

 /usr/local/bin/apl --silent <  ../workspaces/SCRIPT.apl

 or

 /usr/local/bin/apl --silent -f ../workspaces/SCRIPT.apl

   then the first line ‘#! /usr/local/bin/apl --script’ of file
SCRIPT.apl is merely a comment (GNU APL accepts both the traditional APL
character ⍝ and the character # as start of a comment).  The ‘--script’
option is therefore ignored and the following mix of input and output is
shown on the screen.  The input from the script is indented by the usual
APL prompt of 6 blanks, while the output of the APL interpreter is not
indented.

      #! /usr/local/bin/apl --script

      ⎕ARG      ⍝ show command line options
      )OFF      ⍝ leave the interpreter



   If we run the same script directly:

../workspaces/SCRIPT.apl

   Then we get:


 /usr/local/bin/apl --script ../workspaces/SCRIPT.apl


   The ‘--script’ implies ‘--noCIN’ so that the

   input lines for the interpreter are no longer echoed to the output.
That is most likely what you want when writing a script.

   Also, the first line of the script is no longer ignored as a comment,
but controls the command line argument (and thus ⎕ARG) of the
interpreter.  The additional command line argument
‘../workspaces/SCRIPT.apl’ comes from function execve (see ’man 2
execve’).

   If we provide an argument, say SCRIPTARG, to SCRIPT.apl:

../workspaces/SCRIPT.apl SCRIPTARG

   then it shows up at the end of ⎕ARG:


 /usr/local/bin/apl --script ../workspaces/SCRIPT.apl SCRIPTARG


   A final note on scripting in GNU APL is that the ∇-editor works
slightly differently when it is used in a script.  If a user edits an
APL function interactively then an attempt to open an existing function
with a full header gives a DEFN ERROR:


      ∇Z←FOO B
[1] ∇

            ∇Z←FOO B
DEFN ERROR+
      ∇Z←FOO B
             ^
      )MORE
attempt to ∇-open existing function with new function header


   In contrast to a user who can react to the DEFN ERROR, a script
cannot detect this situation and would continue to push lines (which
were intended to be the body of the defined function) into the APL
interpreter.  That would most likely cause a fairly undesirable
behavior.  For example, if the lines of defined function start with line
numbers (like [1], [2], ...)  then every such line would give a SYNTAX
ERROR, and other errors can be expected as well.

   For that reason, if the ∇-editor is used in a script and attempts to
redefine an existing defined function then the existing function is
simply overridden with the new one and no DEFN ERROR is raised.


File: apl.info,  Node: Section 3.2,  Next: Section 3.3,  Prev: Section 3.1,  Up: Chapter 3

3.2 ⎕SYL - System limits
========================

System variable ⎕SYL shows a number of system limits when referenced:

      ⎕SYL
 SI depth limit          (0 = no limit)                    0
 number of values limit  (0 = no limit)                    0
 total ravel bytes limit (0 = no limit)                    0
 current SI depth                                          1
 current number of values                                 54
 current total ravel bytes                              9072
 max. rank for APL values                                  8
 min. ⎕PW                                                 30
 max. ⎕PW                                               1000
 min. ⎕PP                                                  1
 max. ⎕PP                                                 16
 max. input line length                                 2000
 hash table size (obsolete)                            65536
 max. shared variable name length                         64
 max. length of filenames (paths)                       4096
 max. # of shared variables (obsolete)                    64
 max. number of APs                                       16
 max. operators per statement                             16
 largest integer                         9000000000000000000
 smallest integer                       ¯9000000000000000000
 largest numeric exponent                                308
 max. shared variable size (bytes)                     65000
 max. cores (per ./configure)                              0
 max. cores (per max_cores())                              1
 current cores (per core_count())                          1
 print length limit (0 = no limit)                         0

   Most system limits are read-only; attempting to assign ⎕SYL will
result in a SYNTAX ERROR. Indexed assignment to ⎕SYL will result in an
INDEX ERROR unless the limit is writable.  Some read-only limits can be
changed at compile-time via ./configure

3.2.1 ⎕SYL[1 2 3 4 5 6;]
------------------------

The first three limits are writable; their purpose is to stop execution
if:

  1. The SI stack of the interpreter reaches a limit, or
  2. The number of APL values created reaches a limit, or
  3. The memory size of APL values created reaches a limit.

   Whenever a writable limit is reached, a corresponding LIMIT ERROR is
reported and the limit is reset to 0 (meaning no limit) in order to
prevent that the interpreter gets locked up.

   The main use of ⎕SYL is to troubleshoot infinite recursion in
user-defined function that would otherwise cause the machine to start
swapping or to become non-responding.

   Example (stop execution if function call stack exceeds 20):

      ⎕SYL[1;2]←20

   Note that small APL values (up to a configurable number of ravel
elements) count as values (in the number of values limit), but not in
the total ravel bytes limit (the ravel does not allocate memory in
addition to the space already in the number of values limit).  It is
therefore typical to set the number of values limits and the total ravel
bytes limits together.

   An attempt to set a too small limit, for example one that is close to
the current value, is rejected with a DOMAIN ERROR.

3.2.2 ⎕SYL[7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23;]
------------------------------------------------------------

These system limits are read-only.  Some of them can be changed at
compile time via ./configure.  Most others can be changed in the source
code of GNU APL, but doing so is not recommended.

3.2.3 ⎕SYL[24 25 26;]
---------------------

These system limits have been added in preparation of multi-core GNU
APL. They are currently not used.

3.2.4 ⎕SYL[27;]
---------------

This is not a limit that raises a limit error.  Instead it limits the
size of APL values when printed as statement results.  It is fairly easy
in APL to create huge APL values by mistake.  During development it can
be useful to limit the amount of data printed.  If the ravel length of,
say, VALUE exceeds ⎕SYL[27;2] (i.e.  if ⎕SYL[27;2] ≤ ⍴,VALUE) then
SH↑VALUE is printed instead of VALUE. The shape SH is computed like
this:


      SH←⍴VALUE
      while (ravel-length(SH) ≥ ⎕SYL[27;2])   { divide longest axis in SH by 2 }



File: apl.info,  Node: Section 3.3,  Next: Section 3.4,  Prev: Section 3.2,  Up: Chapter 3

3.3 Dyadic ⎕CR
==============

The ⎕CR function has an optional left argument that selects one of
several formatting styles and conversions in addition to the well-known
monadic form.

   Say a byte vector is an integer vector with numbers having a (signed
or unsigned) 8-bit value (i.e.  a value from -128 to 255 inclusive).
Such byte vectors are returned by several ⎕FIO functions and expected as
arguments by several other ⎕FIO function.  Let Z←A ⎕CR B.

   Then the left argument A of ⎕CR selects one of the following:

   A=0-4 or 7-9, or 29: various formatting styles (boxed, APL
input/output, etc.).  Just try them out.

   A=5/6: convert byte vector B to a string of uppercase/lowercase hex
digits.

   A=10: convert variable named in B to an APL expression producing it.

   A=11: convert value B to byte vector Z in CDR format (similar to 3
⎕TF).

   A=12: convert byte vector Z in CDR format to value Z.

   A=13: convert hex string B to byte vector Z.

   If a conversion has an inverse conversion (like 12 being the inverse
of 11) then the inverse conversion can be expressed as the negative of
the conversion number.  For example, 12 ⎕CR B is the same as ¯11 ⎕CR B.

   A=14: conversion 11 followed by conversion 13 (Value to hex string in
CDR format)

   A=15: conversion 13 followed by conversion 12 (hex string in CDR
format to Value)

   A=16: encode byte vector B into Z (base64 encoding, RFC 4648)

   A=17: decode base64 vector B into byte vector Z (base64 encoding, RFC
4648)

   A=18: convert text vector B into byte vector Z (UTF8 encoding, RFC
3629)

   A=19: convert byte vector B into text vector Z (UTF8 encoding, RFC
3629)

   A=20-25: like 3,4,7-9 but using a formatting similar to NARS APL ⎕FMT
(showing the axis lengths as numbers instead of → and ↓)

   A=26: Z is the cell types of the ravel elements of B (2: character,
16: integer, 32: real number, 64: complex number.

   A=27: Z[I] is the primary data representation (for example the real
part of a complex number) of the ravel elements of B[I].

   A=28: Z[I] is the additional data representation (for example the
imaginary part of a complex number) of the ravel elements of B[I].

   A=30: Z is B with all top-level elements conformed to a common rank
and shape (as required by the ⍤ operator)

   Conversions 14 to 17 can be used, for example, to store APL values as
printable ASCII texts in databases.

   For byte vector arguments, a DOMAIN ERROR is thrown if a ravel item
is not a character.  It is, however not checked (for performance
reasons) that the value of the character is in the range -128 to 255.
Instead the low byte of the value is taken.  This is OK for ASCII
strings but does not work for general APL text vectors.  Those need to
be translated to bytes first and can then be used as input for e.g.
byte vector → hex or byte vector → base64 conversions.


File: apl.info,  Node: Section 3.4,  Next: Section 3.5,  Prev: Section 3.3,  Up: Chapter 3

3.4 ⎕FIO: File I/O Functions
============================

As of GNU APL 1.6, the native function FILE_IO has been replaced by the
system function ⎕FIO. See the FILE_IO section below for details.

   A new feature of ⎕FIO (which is not available with the native
function FILE_IO) is ⎕FIO[49].  ⎕FIO[49] is a monadic operator which
takes a monadic conversion function as function argument.  For example:

     Z←F ⎕FIO[49] 'filename'

   reads the file named filename line by line.  For every line read, the
conversion function F is called and the result returned by F is enclosed
and stored in F. In other words,

     Z←F ⎕FIO[49] 'filename'

   does:

     Z F¨Z←⎕FIO[49] 'filename'


File: apl.info,  Node: Section 3.5,  Next: Section 3.6,  Prev: Section 3.4,  Up: Chapter 3

3.5 Debug Commands
==================

Besides the classical commands like )LOAD or )SAVE there are a number of
debug commands that start with ] rather than ).  Regular commands start
with ) and print their output on stdout.  Debug commands start with ]
and print their output on stderr.  Normally you cannot easily
distinguish between stdout and stderr, but another feature, colored
output, uses different colors for stdout and stderr.

   Type )HELP or ]HELP in the interpreter for a list of all commands
available.


File: apl.info,  Node: Section 3.6,  Next: Section 3.7,  Prev: Section 3.5,  Up: Chapter 3

3.6 Colored Output
==================

The APL interpreter gets its input from the standard input (stdin),
which is normally connected to the user’s keyboard, but can also be a
file if APL scripting, the ‘-f’ option, or the ‘-T’ option is used.

   The APL interpreter prints its results on either the standard output
(stdout) for normal APL output, or to the error output (stderr) for
additional trouble-shooting information.

   You can print the 3 channels stdin, stdout, and stderr in different
colors by means of the debug command ]XTERM. Command ]XTERM ON enables
colored output while ]XTERM OFF disables it (for example to avoid
annoying ANSI Escape sequences when forwarding stdout or stderr to a
file).

   By default colored output assumes a terminal (-emulation) that
understands ANSI (or VT100) Escape sequences.  The xterm that comes with
most recent GNU/Linux distributions is a perfect choice supporting both
colors and UTF-8 (Unicode) encoded character I/O.

   Non-ANSI terminals, as well as other colors than the default ones,
can be configured in the ’preferences’ file.  The ’preferences’ file
also contains a description of all possible color settings.


File: apl.info,  Node: Section 3.7,  Next: Section 3.8,  Prev: Section 3.6,  Up: Chapter 3

3.7 Logging Facilities
======================

The APL interpreter has over 30 logging facilities.  Each logging
facility can be ON (and then produces some logging output on stderr) or
OFF. The decision which logging facility shall be ON and which shall be
OFF can be made at compile time (of the APL interpreter) or at run-time.

   If the decision is made at compile time - we call that static logging
- then it cannot be changed later on.  Otherwise - we call that dynamic
logging - there is a debug command ]LOG that allows to turn logging
facilities to be turned ON or OFF.

3.7.1 Static Logging
--------------------

By default the logging facilities that shall be turned ON are defined
statically.  To change the logging facilities that shall be turned ON,
you can edit the file src/Logging.def which defines the different
logging facilities.  The first argument of macro log_def() tells if the
logging facility shall be ON (1) or OFF (0).

   Static logging results in a faster interpreter than dynamic logging
because the decision if something shall be logged or not is made at
compile time and not at run-time.

   If you benchmark the APL interpreter, then ./configure Static Logging
by NOT setting DYNAMIC_LOG_WANTED=yes.

3.7.2 Dynamic Logging
---------------------

Dynamic Logging is intended for trouble-shooters of GNU APL, but also
for those who are interested in the internals of GNU APL. Dynamic
Logging is enabled by setting DYNAMIC_LOG_WANTED=yes when running
./configure.

   If Dynamic Logging is enabled, then the already mentioned file
src/Logging.def determines the initial setting of each logging facility.

   The command

   ]LOG

   (without arguments) then shows all logging facilities and their
current state.  The command

   ]LOG N

   toggles the state of logging facility N from OFF to ON and back.


File: apl.info,  Node: Section 3.8,  Next: Section 3.9,  Prev: Section 3.7,  Up: Chapter 3

3.8 Complex Numbers
===================

Complex numbers are fully supported.


File: apl.info,  Node: Section 3.9,  Next: Section 3.10,  Prev: Section 3.8,  Up: Chapter 3

3.9 Native Functions
====================

A Native Function is a function that can be called in APL like a normal
user defined APL function, but is implemented in C++.

   A native function is created with A ⎕FX B. A is a string that is the
path of a shared library and B is the name of the function in APL.

   The GNU APL package contains a shared library file_io.so that
contains the implementation of a native function for reading and writing
files (fopen(), fclose(), ...), For example:

      ⍝ fix native function in lib_file_io.so as FILE_IO
      ⍝
      'lib_file_io.so' ⎕FX 'FILE_IO'
FILE_IO

      ⍝ show overview of sub-functions in FILE_IO
      ⍝
      FILE_IO ''
   Functions provided by this library.
   Assumes 'lib_file_io.so'  ⎕FX  'FUN'

   Legend: e - error code
           i - integer
           h - file handle (integer)
           s - string
           A1, A2, ...  nested vector with elements A1, A2, ...

           FUN     ''    print this text on stderr
        '' FUN     ''    print this text on stdout
           FUN[ 0] ''    print this text on stderr
        '' FUN[ 0] ''    print this text on stdout

   Zi ←    FUN[ 1] ''    errno (of last call)
   Zs ←    FUN[ 2] Be    strerror(Be)
   Zh ← As FUN[ 3] Bs    fopen(Bs, As) filename Bs mode As
   Zh ←    FUN[ 3] Bs    fopen(Bs, "r") filename Bs
      ...

   Current versions of GNU APL have replaced the native FILE_IO function
above by the sustem function ⎕FIO. ⎕FIO need not be ⎕FX’ed and is
otherwise backward compatible to the native function.  New function
numbers are, however, only added to ⎕FIO and not to the old native
function FILE_IO. The parameters of the functions are described in the
man pages for, e.g.  strerror, fopen, ...  and are fairly obvious.

   Many functions in FILE_IO have byte vectors as arguments or return
byte vectors.  A byte vector is an integer vector whose numbers fit into
a byte (so they are integers between -128 and 255).  Often ⎕UCS and the
functions in dyadic ⎕CR are used to convert such byte vectors to/from,
for example, Unicode strings.

   The GNU APL package also contains other shared libraries as templates
for your own native functions.  Copy one of the files
src/native/template_F0.cc (for niladic native functions),
src/native/template_F12.cc (for nomadic native functions),
src/native/template_OP1.cc (for monadic native operators), or
src/native/template_OP2/cc (for dyadic native operators) to your own .cc
file and adjust src/native/Makefile.am accordingly.

   Note: The )IN and )OUT commands of GNU APL support native functions,
but in order to do so they have to use dyadic ⎕FX. This renders the
workspace interchange file (.atf files) written by )OUT incompatible
with all other APL interpreters if the workspace contains native
functions.  The )OUT command prints a warning when the )OUT command is
used with a workspace that contains native functions.

   Note: As of GNU APL 1.6, the native function FILE_IO has been turned
into the system function ⎕FIO. The syntax of ⎕FIO is the same as for
FILE_IO. The )CLEAR workspace command will close all open files.


File: apl.info,  Node: Section 3.10,  Next: Section 3.11,  Prev: Section 3.9,  Up: Chapter 3

3.10 DUMP Command
=================

GNU APL has a command )DUMP that saves a workspace to disk, similar to
command )OUT). The difference to command )OUT is the file format being
produced.  While )OUT produces a file in IBM’s workspace interchange
format, )DUMP produces a file in GNU APL’s script format (i.e.  readable
APL statements).  Files written with )DUMP can be edited with normal
text editors (vi, Emacs), read back with "apl -f", or made executable
(see scripting).


File: apl.info,  Node: Section 3.11,  Next: Section 3.12,  Prev: Section 3.10,  Up: Chapter 3

3.11 User-defined Commands
==========================

There is a simple mechanism to define additional APL commands.  This
mechanism is intended to introduce new commands by APL libraries.  Like
system commands, user-define commands can only be executed in immediate
execution mode and not from user-defined functions or from ⍎.  It is not
intended to extend the functionality of user-defined commands beyond
what is being described in the following.

   A user-defined command ]NEW_COMMAND is created with the debug command
]USERCMD like this:

      ]USERCMD ]NEW_COMMAND APL_FUNCTION [mode]

   APL_FUNCTION is an APL function that will be called when the command
is entered in immediate execution mode.  The entire line entered by the
user, starting at ]NEW_COMMAND, is the right argument of APL_FUNCTION.
If mode is missing (or 0) then APL_FUNCTION is called monadically.  If
mode is 1 then APL_FUNCTION is called dyadically; the left argument is a
vector of strings that is the left argument broken down into individual
argument strings.

   The function APL_FUNCTION that implements a command need not exist
when the command is created.

   A single user-defined command ]UCMD, or all user-defined commands can
be deleted like this:

      ]USERCMD REMOVE ]UCMD
      ]USERCMD REMOVE-ALL


File: apl.info,  Node: Section 3.12,  Next: Section 3.13,  Prev: Section 3.11,  Up: Chapter 3

3.12 Direct Functions (Lambdas)
===============================

GNU APL supports direct functions (aka.  lambdas), but only in a rather
limited form.

3.12.1 Named Lambdas
--------------------

A statement of the form

      FUN ← { body_statement }

   creates a named lambda.  body_statement can contain variable names ⍺
and ⍵ as well as function names ⍶ and ⍹ which are replaced by the actual
arguments of the lambda.  If both ⍺ and ⍵ are present in body_statement
then the lambda is dyadic.  If only ⍵ is present then it is monadic, and
if neither ⍺ nor ⍵ is present then the lambda is niladic.

   Likewise, if ⍶ and ⍹ are present then the lambda is a dyadic
operator.  If only ⍶ is present then it is a monadic operator, and if
neither ⍶ nor ⍹ is present then the lambda is a normal function.

   GNU APL supports an axis argument in normal user defined functions
and operators.  In lambda expressions the Greek letter χ (Chi) is the
variable name for an axis argument.

   The way a named lambda is implemented in GNU APL is that the
expression

      FUN ← { body_expression }

   is translated to a two-line function ⎕FX ’lambda_header’
’body_expression’.

   For example:

      )FNS

      SUM ← { ⍺ + ⍵ }

      )FNS
SUM

      ∇SUM[⎕]∇
[0] λ←⍺ SUM ⍵
[1] λ← ⍺ + ⍵


   The lambda_header is automatically deduced from the presence or
absence of the variable names (⍺, ⍵, and χ) and function names (⍶ and ⍹)
in the body_expression and from whether the body_expression is empty (no
λ←) or not (with λ←).

   It is possible to specify local variables that work exactly like
their companions in normal defined functions.  Please note that this is
different from some other APL interpreters which treat all variables in
lambda bodies as local variables.  The syntax for specifying local
variables is the same as for the header in normal defined functions:
they are added at the end with semicolons as separators.  In the above
example one could add local variables C and D like this:

      SUM ← { ⍺ + ⍵ ;C;D }

      ⎕CR 'SUM'
λ←⍺ λ1 ⍵;C;D
λ← ⍺ + ⍵


   If a named lambda is created inside a function, then the name of the
lambda (i.e.  the name left of ← { ...  }) can be made a local variable
of the function in which the named lambda is created.  This creates a
lambda with local scope.

   Please note that the above only describes the current implementation
of named lambda in GNU APL. A consequence of that implementation is that
the symbols ⍺, ⍵, ⍶, ⍹, and λ are pretty much behaving like user defined
variables.  In particular, they can be used outside named or unnamed
lambdas.  However, that may change in the future and therefore such use
of ⍺, ⍵, ⍶, ⍹, and λ outside is certainly a bad idea.

3.12.2 Unnamed Lambdas
----------------------

An unnamed lambda is an expression inside { } but without assigning it
to a name.  This is often used together with the EACH operator.  For
example:


      { ⍴ , ⍵ } ¨ 'a' 'ab' 'abc'
 1  2  3


   Unnamed lambdas are automatically local in scope (similar to labels).
They can be passed as function arguments to operators.  However, unnamed
lambdas are NOT inserted into the symbol table of the interpreter.  They
are therefore not visible to functions like ⎕CR or by the ∇-editor.
Occasionally the names λ1, λ2, ...  may show up in commands like )SIS.
These names are automatically generated for unnamed lambdas in order to
provide a name in places where a function name is needed.

3.12.3 Limitations of Lambdas
-----------------------------

There are a number of features related to lambdas that are present in
other APL interpreters but that are NOT implemented in GNU APL. This
includes multiple statements, guards, lexical scoping, and probably
more.


File: apl.info,  Node: Section 3.13,  Next: Section 3.14,  Prev: Section 3.12,  Up: Chapter 3

3.13 Axis argument in defined functions
=======================================

Defined functions and operators (including lambdas) accept an axis
argument.  For example:


∇Z←Average[X] B
 Z←(+/[X]B) ÷ (⍴B)[X]
∇

      Average[1] 5 5⍴⍳25
11 12 13 14 15

      Average[2] 5 5⍴⍳25
3 8 13 18 23


   Syntactically, the axis is used in the same way as for primitive
functions and operators.

   There are no constraints on the axis such as being integers.
Therefore you can use an axis as a third function argument.  Keep in
mind, however, that doing so will make your APL code incompatible with
other APL interpreters.  Use this feature carefully!


File: apl.info,  Node: Section 3.14,  Next: Section 3.15,  Prev: Section 3.13,  Up: Chapter 3

3.14 ⎕SI - State Indicator
==========================

⎕SI returns aspects of the current State Indicator, similar to the
standard command )SI. This can be used, for example, to create debug
functions similar to the assert() macro in C/C++:


∇Assert B;COND;LOC;VAR
 →(1≡B)⍴0
 ' '
 COND←7↓,¯2 ⎕SI 4
 LOC←,¯2 ⎕SI 3
 '************************************************'
 ' '
 '*** Assertion (', COND, ') failed at ',LOC
 ''

 ⍝ show stack
 ⍝
 ' '
 'Stack:'
 7 ⎕CR ⊃¯1↓⎕SI 3
 ' '
 '************************************************</pre>'
 →
∇


   ⎕SI is a system function whose right argument specifies which aspect
of the State indicator shall be returned:

   ⎕SI 1: The name of the context.  That is either the name of a user
defined function, or ◊ for an immediate execution context, or ⍎ for an
execute context.

   ⎕SI 2: The line number (of a defined function) or 0 for immediate
execution and execute.

   ⎕SI 3: The function name and line number in square brackets, like
foo[3]

   ⎕SI 4: Either the statement text of the function line or the error
text of an error that has occurred on the line

   ⎕SI 5: The program counter (= token number counted from the start of
the function text

   ⎕SI 6: the parse mode of the context (immediate execution, execute,
or defined function).

   If no left argument is provided then the result of ⎕SI is a vector
with one entry per State Indicator level (and hence ⍴⎕SI B is the depth
of the SI).

   If a left argument A is provided then specifies a particular level of
the SI instead of the entire SI. A should be an integer scalar.  If A is
positive then the level is counted from the oldest entry to the latest,
while negative A count from the latest to the oldest level.

   For example, ¯1 ⎕SI refers to the currently executing context, ¯2 ⎕SI
is the caller, and so on.


File: apl.info,  Node: Section 3.15,  Next: Section 3.16,  Prev: Section 3.14,  Up: Chapter 3

3.15 ⎕DLX - Knuth’s Dancing Links Algorithm
===========================================

⎕DLX is an implementation of Donald Knuth’s Dancing Links Algorithm
(called DLX by Knuth himself, but is sometimes also referred to as
Knuth’s Algorithm X) ⎕DLX is a generic backtracking machinery that can
be used to dramatically simplify problems like the 8 queens problem on a
chess board or sudokus.

   The monadic form of ⎕DLX, i.e.  ⎕DLX B, is a shortcut for 0 ⎕DLX B.
It computes the first solution for the constraint matrix B.

   The dyadic form of ⎕DLX, i.e.  A ⎕DLX B, has a integer scalar A as
left argument which determines the details of the computation as
follows:

   A > 0: The algorithm tries to find all solutions, but stops when A
solutions have been found.  This is handy while debugging code using
⎕DLX. The result is a nested vector with one vector item per solution.

   A = 0: The algorithm stops when the first solution was found.  In
this case the solution is a simple (non-nested) numeric vector.

   A = ¯1: like A > 0 but finding all solutions

   A = ¯2: like A = 0 but instead of returning the first solution, the
number of solutions (i.e.  0 or 1), the number of backtracks, and the
number of link dances is returned as a 3-element numeric vector.

   A = ¯3: like A = ¯1 but instead of returning all solutions, the
number of solutions, the number of backtracks, and the number of link
dances is returned as a 3-element numeric vector.

   A = ¯4: A number of single step in Knuth’s Algoritm are performed.
Let e.g.  A←¯4 r1 r2 r3.  Then Z←A ⎕DLX B is the matrix B after 3 steps
r1, r2, and r3 have been performed.  r1, r2, and r3 are valid (as per
⎕IO) row numbers of B, and a step with a given row changes B as follows:
   • the given row is set to 0 (this row becomes part of the final
     result),
   • all other rows that have a 1 or a 2 in the same column as B are set
     to 0 (these 1s or 2s in the other rows conflict with the 1 or 2 in
     the given row and are therefore reemoved from B), and
   • all columns that have no 1s or 2s left are removed from B (these
     columns habe a 1 or 2 in B and the constraint is therefore
     satisfied).
   The prupose of ¯4 ⎕DLX is:
   • To demonstrate how Knuth’s algorithm works, and/or
   • To preset some initial values in B. For example one can first
     initialize the contraints for an empty Sudoku (which is the same
     matrix for all Sudokus) and then enter the initial digits of a
     particular Sudokus using 4 ⎕DLX. If thre are, say, 20 initial
     digits in the Sudoku then A has 21 elements (¯4 plus the 20 rows
     representing the 20 initial digits.  The result of ¯4 ⎕DLX B has
     the same number of rows as B (with some rows now cleared), but
     fewer columns than B.

   The right argument B of ⎕DLX B or A ⎕DLX B is a constraints matrix
whose columns consist of either 0s and 1s (called a primary column) or
0s and 2s (called secondary columns).  The 0s, 1s and 2s can be the
integers 0, 1, or 2, characters ’0’, ’1’, or ’2’ respectively, or ’ ’
meaning ’0’.  The character representation is useful when B becomes
large and shall be printed because the spaces in the numerical variant
will not be printed.  In the following, B is assumed to be numeric.

   Let Z←A ⎕DLX B, and let R be a solution in Z, that is, Z itself (A =
0) or R is Z[k] for some k if A ≠ 0.  And let S←+⌿B[R].

   Then S = s1 s2 s3, ...  sN where N is the number of columns in B and
sj=1 if j is a primary column of B and sj∈0 1 if j is a secondary column
of B.

   In other words, ⎕DLX B computes a subset of the rows of B in such a
way that for every column j of B exactly one (for primary column j) or
at most one (for a secondary column) of the rows in a solution has its
j’th element set to 1 and all other words set their j’th element set to
0.

   In yet other words, for every solution returned by ⎕DLX B, a 1 in one
row prevents all other rows that also have a 1 in that column, and all
rows together have exactly one 1 in every primary column and at most one
1 in every secondary column.  In the absence of secondary columns, the
problem solved by ⎕DLX is also known as the "exact cover problem"

   If all that sounds weird and useless, consider the following APL
program for finding all solutions of the 8 Queens problem on a chess
board (which probably every programmer has programmed at some point in
time):

      RC←8↑'1' ◊ D←15↑¯8↑'2'   ⍝ helpers for constructing Q8
      ⍝     rows   cols   diag1    diag2
      Q8←⊃{(R⌽RC),(C⌽RC),((C-R)⌽D),((¯7-R+C)⌽D)⊣(R C)←-8 8⊤⍵-⎕IO} ¨ ⍳64
      Z←¯1 ⎕DLX Q8
      {⎕UCS (65+⌊⍵÷8)(49+8∣⍵←⍵-⎕IO)} ¨ ⊃Z[1 2 3 92] ⍝ solutions 1, 2, and 92
 A1 B6 C8 D3 E7 F4 G2 H5
 A1 C5 F4 H3 D8 G6 B7 E2
 A8 D6 H4 G7 C1 F5 B3 E2


   Obviously Z contains solutions for the 8 Queens problem; the total
number of solutions is well known to be 92 and we showed only the first
two and the last solution above.

   The constraint matrix Q8 is the key to success.  The matrix has 64
rows - one row for every field of the chess board.  And it has 8 + 8 +
15 + 15 columns.  The first 8 columns of Q8 are constraints that prevent
more than one Queen to be placed in the same row of the chess board (the
argument ⍵ is the field number counting from left to right and from
bottom to top).  The next 8 columns of Q8 are constraints that prevent
more than one Queen to be placed in the same column of the chess board.

   If we would call ⎕DLX with only these constraints, i.e.  ⎕DLX T8←64
16↑Q8, then we would get the solutions of the 8 tower problem.  However,
we continue and add 15 more constraints for each of the two diagonals.
The resulting constraint matrix Q8 is this:

      Q8

1       1              2                     2
1        1              2                   2
1         1              2                 2
1          1              2               2
1           1              2             2
1            1              2           2
1             1              2         2
1              1              2       2
 1      1             2                     2
 1       1             2                   2
 1        1             2                 2
 1         1             2               2
 1          1             2             2
 1           1             2           2
 1            1             2         2
 1             1             2       2
  1     1            2                     2
  1      1            2                   2
  1       1            2                 2
  1        1            2               2
  1         1            2             2
  1          1            2           2
  1           1            2         2
  1            1            2       2
   1    1           2                     2
   1     1           2                   2
   1      1           2                 2
   1       1           2               2
   1        1           2             2
   1         1           2           2
   1          1           2         2
   1           1           2       2
    1   1          2                     2
    1    1          2                   2
    1     1          2                 2
    1      1          2               2
    1       1          2             2
    1        1          2           2
    1         1          2         2
    1          1          2       2
     1  1         2                     2
     1   1         2                   2
     1    1         2                 2
     1     1         2               2
     1      1         2             2
     1       1         2           2
     1        1         2         2
     1         1         2       2
      1 1        2                     2
      1  1        2                   2
      1   1        2                 2
      1    1        2               2
      1     1        2             2
      1      1        2           2
      1       1        2         2
      1        1        2       2
       11       2                     2
       1 1       2                   2
       1  1       2                 2
       1   1       2               2
       1    1       2             2
       1     1       2           2
       1      1       2         2
       1       1       2       2

   To see how, for example, the first solution looks like and how it
relates to the constraints matrix Q8:


      ⍝ the rows in Q8 of the first solution
      ⍝
      Z[1]
 1 14 24 27 39 44 50 61

      ⍝ the first solution translated back into the problem domain
      ⍝
      {⎕UCS (65+⌊⍵÷8)(49+8∣⍵←⍵-⎕IO)} ¨ ⊃Z[1]
 A1 B6 C8 D3 E7 F4 G2 H5

      ⍝ the constraints of the rows of the first solution
      ⍝
      Q8[⊃Z[1];]
1       1              2                     2
 1           1             2           2
  1            1            2       2
   1      1           2                 2
    1         1          2         2
     1     1         2               2
      1  1        2                   2
       1    1       2             2

      ⍝ all primary contraints met?
      ⍝
      +⌿ ' '≠ Q8[⊃Z[1];]
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 0 1 1 1 1 0 1 0 1 1 0 0 0 0 0 1 1 1 1 1 1
      1 0 0 0 0 1


   And that’s it: call ⎕DLX to get the solution(s).  In general all
problems that can be solved by ⎕DLX contain 3 steps:

   • translate the problem into a constraints matrix B,
   • Z←⎕DLX B, and
   • translate the result Z back into the problem domain.

   Another application of ⎕DLX is solving sudokus.  The constraints
matrix is a little more complicated, but the principle is the same.


File: apl.info,  Node: Section 3.16,  Next: Section 3.17,  Prev: Section 3.15,  Up: Chapter 3

3.16 History and TAB completion
===============================

Until GNU APL 1.4 / SVN 465, GNU APL used libreadline for interactive
user input.  libreadline did provide two useful features: tab expansion
(the tab key would expand file names) and history (the cursor up/down
keys would recall previously entered lines).

   Since SVN 465 libreadline was removed and the standard TAB expansion
and history of libreadline were replaced by more context sensitive (i.e.
APL aware) implementations:

   1.  Instead of simply recalling the last line entered by the user,
there are now different histories for different input contexts:

   1a.  The input history in immediate execution recalls the last line
entered in immediate execution (and not, for example, lines entered in
function editing mode or ⍞ input.

   1b.  Likewise, ⍞ recalls the last line entered for ⍞-input

   1c.  ⎕ recalls the last line entered for ⎕-input

   1d.  In the ∇-editor, the other function lines of the function being
edited can be recalled.  This is far more handy than the ∇-editor
commands for recalling function lines (and which are not fully supported
in GNU APL).

   2.  Instead of always TAB-completing file names, the tab character
now understands different TAB-completion contexts:

   2a.  Input starting with .  or / is completed as a filename like
readline did.

   2b.  Input starting with ) or ] is completed as command name name or,
to some extent, as command arguments.

   2c.  Input starting with ⎕ is completed as a system function name or
a system variable name.

   2d.  Input starting with letters, ∆, or ⍙ is completed as a user
defined function or variable name.


File: apl.info,  Node: Section 3.17,  Next: Section 3.18,  Prev: Section 3.16,  Up: Chapter 3

3.17 ⎕SQL - SQL Database Interface
==================================

As of GNU APL 1.6, the native function SQL has been replaced by the
system function ⎕SQL, described below.

      ⎕sql[0]''
Available function numbers:
type  ⎕SQL[1] file      - open a database file, return reference ID for it
      ⎕SQL[2] ref       - close database
query ⎕SQL[3,db] params - send SQL query
query ⎕SQL[4,db] params - send SQL update
      ⎕SQL[5] ref       - begin a transaction
      ⎕SQL[6] ref       - commit current transaction
      ⎕SQL[7] ref       - rollback current transaction
      ⎕SQL[8] ref       - list tables
ref   ⎕SQL[9] table     - list columns for table

      type  ⎕SQL[1] file

   Connect to database of type L using connection arguments R.

   L must be a string indicating the database type.  Current supported
values are ’postgresql’ and ’sqlite’.

   R is the connection parameters which depends on the type of database:

   - For type≡’sqlite’: the argument is string pointing to the database
file.

   - For type≡’postgresql’, the argument is a standard connect string as
described in the PostgreSQL documentation.

   This function returns a database handle that should be used when
using other SQL functions.  This value should be seen as an opaque
handle.  It is, however, guaranteed that the handle is a scalar value.

      ⎕SQL[2] ref

   Disconnect from database R.

   R is the database handle that should be disconnected.  After this
function has been called, no further operations are to be performed on
this handle.  Future calls to SQL∆Connect may reuse previously
disconnected handles.

      query ⎕SQL[3,db] params

   Execute a select statement and return the result table.

   The axis parameter indicates the database handle.

   L is a select statement to be executed.  Positional parameters can be
supplied by specifying a question mark "?"  in the statemement.

   R is an array containing the values for the positional parameters.
If the array is of rank 2, the statement will be executed multiple times
with each row being the values for each call.

   The return value is a rank-2 array representing the result of the
select statement.  Null values are returned as ⍬ and empty strings are
returned as ”.

      query ⎕SQL[4,db] params

   Execute an SQL statement that does not return a result.

   This function is identical to SQL∆Select with the exception that it
is used on statements which do not return a result table.

      ⎕SQL[5] ref

   Begin a transaction.

      ⎕SQL[6] ref

   Commit a transaction.

      ⎕SQL[7] ref

   Rolls back the current transaction.

      ⎕SQL[8] ref

   Return an array containing the name of all tables.

      ref   ⎕SQL[9] table

   Return an array containing information about the columns in the given
table.  Currently, the column layout is as follows:

   Name Type

   More columns containing extra information may be added in a future
release.


File: apl.info,  Node: Section 3.18,  Prev: Section 3.17,  Up: Chapter 3

3.18 Comparison Rules
=====================

Both IBM APL2 and the ISO standard require that the arguments of <, ≤,
≥, and > (but not = or ≠) are integer or real numbers, and that the
argument of monadic ⍋ or ⍒ is a vector of integer or real numbers.  In
contrast GNU APL also allows the comparison of characters and numbers or
the comparison of complex numbers according to the following, more
general, rules.

   Let A and B two APL values to be compared.  The first comparison
below that succeeds (i.e.  returns a result) is then the result of
comparing A and B.

   1.  If (⍴⍴A) < (⍴⍴B) then A < B. And vice versa.

   2.  If (⍴A) < (⍴B) then A < B. And vice versa.

   3.  Otherwise ⍴A and ⍴B are equal.  If all corresponding ravel
elements of A and B are equal (i.e.  tolerantly equal within ⎕CT as
defined in the ISO standard) then A = B. Otherwise let A1 and B1 the
first corresponding ravel elements of A and B with A1≠B1.

   4.  If A1 and B1 are both character values and (⎕UCS A1) < (⎕UCS B1)
then A < B. And vice versa.

   5.  If A1 is a character and B1 is numeric, then A < B. And vice
versa.

   6.  If A1 and B1 are both numeric values then:

   6a.  If (9○A1) < (9○B1) then A < B. And vice versa.

   6a.  Otherwise if (11○A1) < (11○B1) then A < B. And vice versa.

   Another way of describing the rules above is that the comparison of
two values is comprised of sub-comparisons of certain properties of the
values in the following order:

   • ranks of the values
   • shapes of the values
   • type (character vs.  numeric) of the values,
   • unicodes of character values,
   • real parts of numeric values,
   • imaginary parts of numeric values,

   Note: Rules 1.  and 2.  above are only relevant for comparisons made
in the context of sorting (i.e.  for ⍋ or ⍒).  This is because for =, ≠,
<, ≤, ≥, or > either a RANK ERROR or a LENGTH ERROR is raised if the
ranks or shapes of A and B do not match:

      (9 8) < (1 2 3)
LENGTH ERROR
      9 8<1 2 3
      ^  ^

      ⍋(9 8) (1 2 3)
1 2

   The reason for comparing complex numbers first by their real parts
and then by their imaginary part and not, for example, first by their
magnitude and then by their angle is that the chosen order gives more
consistent results when comparing near-complex numbers or their true
real compagnions.  For example, a magnitude first comparison of complex
numbers would make ¯1 < ¯2J∈ for a near-complex ¯2J∈ but ¯1 > ¯2 for a
true real ¯2.


File: apl.info,  Node: Chapter 4,  Next: Chapter 5,  Prev: Chapter 3,  Up: Top

4 Limitations and Missing Features
**********************************

APL is an almost full implementation of the ISO standard 13751.
However, a few limitations exist.  See file README-4-compliance for an
up-to-date descriptions of limitations.

4.1 System Limits
=================

APL integers are 64-bit wide, ranging from -9200000000000000000 to
9200000000000000000

   Due to rounding and similar, some integers can become slightly
larger, but the above limits are guaranteed for some operations (e.g.  +
and -)

   APL floating point values are 64-bit wide, thus ranging from
-8.98E307 to 8.98E307.

   APL values have a maximum rank of 8.  This value can be increased,
however, by means of ./configure (see README-2-configure).

4.2 Shared Variables
====================

The system functions and variables for shared variables, i.e.  ⎕SVO,
⎕SVR, ⎕SVC, ⎕SVS, ⎕SVQ are implemented.

   Communication between two workspaces is supported, but with some
limitations.  Two workspaces must run on the same machine in order to
communicate via shared variables.

   However, only two auxiliary processors, AP100 and AP210, are provided
as examples for implementing auxiliary processors.

   Shared variables for auxiliary processors are provided for some
backward compatibility.  In the past - without access to the source code
of the APL interpreter - shared variables were the only method available
to extend the functionality of the interpreter.

   With this interpreter - and access to its source code - it is often
more convenient to add your own commands or your own system variables to
the APL interpreter rather that adding auxiliary processors.


File: apl.info,  Node: Chapter 5,  Next: Chapter 6,  Prev: Chapter 4,  Up: Top

5 Internationalization
**********************

GNU APL used to be internationalized by means of GNU gettext, but is not
anymore.

   For APL characters to work properly with keyboards and terminal
emulators (see also README-3-keyboard) it is essential that your LANG
environment variable is correctly set.  For German, that would be
"de_DE.utf8", like:

   $export LANG=de_DE.utf8

   A misspelling of the language can cause APL characters to become
unavailable, even if your xmodmap is correct.  You can check your
setting by executing the program ’locale’.  If it shows warnings like
these:

   locale: Cannot set LC_CTYPE to default locale: No such file or
directory locale: Cannot set LC_MESSAGES to default locale: No such file
or directory locale: Cannot set LC_ALL to default locale: No such file
or directory

   then something is wrong and you should check your LANG setting and
installed locales.

   As far as xterm is concerned, the xterm command line options –u8,
–lc, and –en might solve problems related to the terminal.


File: apl.info,  Node: Chapter 6,  Next: Chapter 7,  Prev: Chapter 5,  Up: Top

6 Libraries Contributed by GNU APL Users
****************************************

Since the first release of GNU APL, a number of libraries, but also
other developments, have been made by users of GNU APL. Below are links
to these contributions.

* Menu:

* Section 6.1::  The GNU APL Community Webpage
* Section 6.2::  Core Libraries


File: apl.info,  Node: Section 6.1,  Next: Section 6.2,  Up: Chapter 6

6.1 The GNU APL Community Webpage
=================================

The GNU APL project maintains a web page that lists contributions made
by GNU APL users:

   <http://www.gnu.org/software/apl/Community.html>

   The contributions listed there were collected from emails sent to
bug-apl@gnu.org

   Since the list was started long after bug-apl@gnu.org was created.
It is therefore quite possible that your contribution is missing.  That
does not mean that your contribution was not considered worthwhile to be
listed but more likely that it was overlooked.  Just send an email to
bug-apl@gnu.org to fix that.


File: apl.info,  Node: Section 6.2,  Prev: Section 6.1,  Up: Chapter 6

6.2 Core Libraries
==================

There is a small number of libraries that are considered "core" APL
because, for example, the ISO standard requires their functionality, or
because other APL interpreters normally support them.

   These libraries are linked via the GNU APL Community web page.  In
addition they are stored in the GNU APL SVN repository and also included
in the GNU APL packages that follows their contribution.

   Currently the following core libraries exist:

   GNU APL Emacs mode (for Emacs users) by Elias Mårtenson

   SQL interface (SQLite and Postgres) by Elias Mårtenson

   Component File System (as required by ISO 13751) by David B. Lamkins

   Component File System (as required by ISO 13751) by Blake McBride

   SQL


File: apl.info,  Node: Chapter 7,  Prev: Chapter 6,  Up: Top

7 Licenses for this GNU APL Manual and for GNU APL
**************************************************

This manual for GNU APL is licensed under the GNU Free Documentation
License, *Note Section 4.1:: below.

   GNU APL itself is licensed under the GNU Public License version 3 or
later (aka GPLv3+), *Note Section 4.2:: below.

* Menu:

* Section 7.1::  The GNU Free Documentation License
* Section 7.2::  The GNU General Public License


File: apl.info,  Node: Section 7.1,  Next: Section 7.2,  Up: Chapter 7

7.1 The GNU Free Documentation License
======================================

                     Version 1.3, 3 November 2008

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of “copyleft”, which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
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     of the public is a licensee, and is addressed as “you”.  You accept
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     A “Modified Version” of the Document means any work containing the
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     The “Invariant Sections” are certain Secondary Sections whose
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     been arranged to thwart or discourage subsequent modification by
     readers is not Transparent.  An image format is not Transparent if
     used for any substantial amount of text.  A copy that is not
     “Transparent” is called “Opaque”.

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The “Title Page” means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, “Title
     Page” means the text near the most prominent appearance of the
     work’s title, preceding the beginning of the body of the text.

     The “publisher” means any person or entity that distributes copies
     of the Document to the public.

     A section “Entitled XYZ” means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.)
     To “Preserve the Title” of such a section when you modify the
     Document means that it remains a section “Entitled XYZ” according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow the
     conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document’s license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the title
     equally prominent and visible.  You may add other material on the
     covers in addition.  Copying with changes limited to the covers, as
     long as they preserve the title of the Document and satisfy these
     conditions, can be treated as verbatim copying in other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a machine-readable
     Transparent copy along with each Opaque copy, or state in or with
     each Opaque copy a computer-network location from which the general
     network-using public has access to download using public-standard
     network protocols a complete Transparent copy of the Document, free
     of added material.  If you use the latter option, you must take
     reasonably prudent steps, when you begin distribution of Opaque
     copies in quantity, to ensure that this Transparent copy will
     remain thus accessible at the stated location until at least one
     year after the last time you distribute an Opaque copy (directly or
     through your agents or retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of copies,
     to give them a chance to provide you with an updated version of the
     Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with the
     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
          versions (which should, if there were any, be listed in the
          History section of the Document).  You may use the same title
          as a previous version if the original publisher of that
          version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document’s
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled “History”, Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on the
          Title Page.  If there is no section Entitled “History” in the
          Document, create one stating the title, year, authors, and
          publisher of the Document as given on its Title Page, then add
          an item describing the Modified Version as stated in the
          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in the
          “History” section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled “Acknowledgements” or “Dedications”,
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
          in their text and in their titles.  Section numbers or the
          equivalent are not considered part of the section titles.

       M. Delete any section Entitled “Endorsements”.  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          “Endorsements” or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option designate
     some or all of these sections as invariant.  To do this, add their
     titles to the list of Invariant Sections in the Modified Version’s
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled “Endorsements”, provided it contains
     nothing but endorsements of your Modified Version by various
     parties—for example, statements of peer review or that the text has
     been approved by an organization as the authoritative definition of
     a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end of
     the list of Cover Texts in the Modified Version.  Only one passage
     of Front-Cover Text and one of Back-Cover Text may be added by (or
     through arrangements made by) any one entity.  If the Document
     already includes a cover text for the same cover, previously added
     by you or by arrangement made by the same entity you are acting on
     behalf of, you may not add another; but you may replace the old
     one, on explicit permission from the previous publisher that added
     the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     “History” in the various original documents, forming one section
     Entitled “History”; likewise combine any sections Entitled
     “Acknowledgements”, and any sections Entitled “Dedications”.  You
     must delete all sections Entitled “Endorsements.”

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the documents
     in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an “aggregate” if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation’s users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document’s Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled “Acknowledgements”,
     “Dedications”, or “History”, the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License “or any later version” applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy’s public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     “Incorporate” means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is “eligible for relicensing” if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the “with…Texts.” line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.


File: apl.info,  Node: Section 7.2,  Prev: Section 7.1,  Up: Chapter 7

7.2 The GNU GENERAL PUBLIC LICENSE
==================================

GNU GENERAL PUBLIC LICENSE Version 3, 29 June 2007

   Copyright (C) 2007 Free Software Foundation, Inc.  <http://fsf.org/>
Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.

   Preamble

   The GNU General Public License is a free, copyleft license for
software and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program–to make sure it remains free
software for all its users.  We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to
any other work released this way by its authors.  You can apply it to
your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.

   To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights.  Therefore, you have
certain responsibilities if you distribute copies of the software, or if
you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
freedoms that you received.  You must make sure that they, too, receive
or can get the source code.  And you must show them these terms so they
know their rights.

   Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.

   For the developers’ and authors’ protection, the GPL clearly explains
that there is no warranty for this free software.  For both users’ and
authors’ sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.

   Some devices are designed to deny users access to install or run
modified versions of the software inside them, although the manufacturer
can do so.  This is fundamentally incompatible with the aim of
protecting users’ freedom to change the software.  The systematic
pattern of such abuse occurs in the area of products for individuals to
use, which is precisely where it is most unacceptable.  Therefore, we
have designed this version of the GPL to prohibit the practice for those
products.  If such problems arise substantially in other domains, we
stand ready to extend this provision to those domains in future versions
of the GPL, as needed to protect the freedom of users.

   Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
software on general-purpose computers, but in those that do, we wish to
avoid the special danger that patents applied to a free program could
make it effectively proprietary.  To prevent this, the GPL assures that
patents cannot be used to render the program non-free.

   The precise terms and conditions for copying, distribution and
modification follow.

   TERMS AND CONDITIONS

   0.  Definitions.

   "This License" refers to version 3 of the GNU General Public License.

   "Copyright" also means copyright-like laws that apply to other kinds
of works, such as semiconductor masks.

   "The Program" refers to any copyrightable work licensed under this
License.  Each licensee is addressed as "you".  "Licensees" and
"recipients" may be individuals or organizations.

   To "modify" a work means to copy from or adapt all or part of the
work in a fashion requiring copyright permission, other than the making
of an exact copy.  The resulting work is called a "modified version" of
the earlier work or a work "based on" the earlier work.

   A "covered work" means either the unmodified Program or a work based
on the Program.

   To "propagate" a work means to do anything with it that, without
permission, would make you directly or secondarily liable for
infringement under applicable copyright law, except executing it on a
computer or modifying a private copy.  Propagation includes copying,
distribution (with or without modification), making available to the
public, and in some countries other activities as well.

   To "convey" a work means any kind of propagation that enables other
parties to make or receive copies.  Mere interaction with a user through
a computer network, with no transfer of a copy, is not conveying.

   An interactive user interface displays "Appropriate Legal Notices" to
the extent that it includes a convenient and prominently visible feature
that (1) displays an appropriate copyright notice, and (2) tells the
user that there is no warranty for the work (except to the extent that
warranties are provided), that licensees may convey the work under this
License, and how to view a copy of this License.  If the interface
presents a list of user commands or options, such as a menu, a prominent
item in the list meets this criterion.

   1.  Source Code.

   The "source code" for a work means the preferred form of the work for
making modifications to it.  "Object code" means any non-source form of
a work.

   A "Standard Interface" means an interface that either is an official
standard defined by a recognized standards body, or, in the case of
interfaces specified for a particular programming language, one that is
widely used among developers working in that language.

   The "System Libraries" of an executable work include anything, other
than the work as a whole, that (a) is included in the normal form of
packaging a Major Component, but which is not part of that Major
Component, and (b) serves only to enable use of the work with that Major
Component, or to implement a Standard Interface for which an
implementation is available to the public in source code form.  A "Major
Component", in this context, means a major essential component (kernel,
window system, and so on) of the specific operating system (if any) on
which the executable work runs, or a compiler used to produce the work,
or an object code interpreter used to run it.

   The "Corresponding Source" for a work in object code form means all
the source code needed to generate, install, and (for an executable
work) run the object code and to modify the work, including scripts to
control those activities.  However, it does not include the work’s
System Libraries, or general-purpose tools or generally available free
programs which are used unmodified in performing those activities but
which are not part of the work.  For example, Corresponding Source
includes interface definition files associated with source files for the
work, and the source code for shared libraries and dynamically linked
subprograms that the work is specifically designed to require, such as
by intimate data communication or control flow between those subprograms
and other parts of the work.

   The Corresponding Source need not include anything that users can
regenerate automatically from other parts of the Corresponding Source.

   The Corresponding Source for a work in source code form is that same
work.

   2.  Basic Permissions.

   All rights granted under this License are granted for the term of
copyright on the Program, and are irrevocable provided the stated
conditions are met.  This License explicitly affirms your unlimited
permission to run the unmodified Program.  The output from running a
covered work is covered by this License only if the output, given its
content, constitutes a covered work.  This License acknowledges your
rights of fair use or other equivalent, as provided by copyright law.

   You may make, run and propagate covered works that you do not convey,
without conditions so long as your license otherwise remains in force.
You may convey covered works to others for the sole purpose of having
them make modifications exclusively for you, or provide you with
facilities for running those works, provided that you comply with the
terms of this License in conveying all material for which you do not
control copyright.  Those thus making or running the covered works for
you must do so exclusively on your behalf, under your direction and
control, on terms that prohibit them from making any copies of your
copyrighted material outside their relationship with you.

   Conveying under any other circumstances is permitted solely under the
conditions stated below.  Sublicensing is not allowed; section 10 makes
it unnecessary.

   3.  Protecting Users’ Legal Rights From Anti-Circumvention Law.

   No covered work shall be deemed part of an effective technological
measure under any applicable law fulfilling obligations under article 11
of the WIPO copyright treaty adopted on 20 December 1996, or similar
laws prohibiting or restricting circumvention of such measures.

   When you convey a covered work, you waive any legal power to forbid
circumvention of technological measures to the extent such circumvention
is effected by exercising rights under this License with respect to the
covered work, and you disclaim any intention to limit operation or
modification of the work as a means of enforcing, against the work’s
users, your or third parties’ legal rights to forbid circumvention of
technological measures.

   4.  Conveying Verbatim Copies.

   You may convey verbatim copies of the Program’s source code as you
receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice; keep
intact all notices stating that this License and any non-permissive
terms added in accord with section 7 apply to the code; keep intact all
notices of the absence of any warranty; and give all recipients a copy
of this License along with the Program.

   You may charge any price or no price for each copy that you convey,
and you may offer support or warranty protection for a fee.

   5.  Conveying Modified Source Versions.

   You may convey a work based on the Program, or the modifications to
produce it from the Program, in the form of source code under the terms
of section 4, provided that you also meet all of these conditions:

   a) The work must carry prominent notices stating that you modified
it, and giving a relevant date.

   b) The work must carry prominent notices stating that it is released
under this License and any conditions added under section 7.  This
requirement modifies the requirement in section 4 to "keep intact all
notices".

   c) You must license the entire work, as a whole, under this License
to anyone who comes into possession of a copy.  This License will
therefore apply, along with any applicable section 7 additional terms,
to the whole of the work, and all its parts, regardless of how they are
packaged.  This License gives no permission to license the work in any
other way, but it does not invalidate such permission if you have
separately received it.

   d) If the work has interactive user interfaces, each must display
Appropriate Legal Notices; however, if the Program has interactive
interfaces that do not display Appropriate Legal Notices, your work need
not make them do so.

   A compilation of a covered work with other separate and independent
works, which are not by their nature extensions of the covered work, and
which are not combined with it such as to form a larger program, in or
on a volume of a storage or distribution medium, is called an
"aggregate" if the compilation and its resulting copyright are not used
to limit the access or legal rights of the compilation’s users beyond
what the individual works permit.  Inclusion of a covered work in an
aggregate does not cause this License to apply to the other parts of the
aggregate.

   6.  Conveying Non-Source Forms.

   You may convey a covered work in object code form under the terms of
sections 4 and 5, provided that you also convey the machine-readable
Corresponding Source under the terms of this License, in one of these
ways:

   a) Convey the object code in, or embodied in, a physical product
(including a physical distribution medium), accompanied by the
Corresponding Source fixed on a durable physical medium customarily used
for software interchange.

   b) Convey the object code in, or embodied in, a physical product
(including a physical distribution medium), accompanied by a written
offer, valid for at least three years and valid for as long as you offer
spare parts or customer support for that product model, to give anyone
who possesses the object code either (1) a copy of the Corresponding
Source for all the software in the product that is covered by this
License, on a durable physical medium customarily used for software
interchange, for a price no more than your reasonable cost of physically
performing this conveying of source, or (2) access to copy the
Corresponding Source from a network server at no charge.

   c) Convey individual copies of the object code with a copy of the
written offer to provide the Corresponding Source.  This alternative is
allowed only occasionally and noncommercially, and only if you received
the object code with such an offer, in accord with subsection 6b.

   d) Convey the object code by offering access from a designated place
(gratis or for a charge), and offer equivalent access to the
Corresponding Source in the same way through the same place at no
further charge.  You need not require recipients to copy the
Corresponding Source along with the object code.  If the place to copy
the object code is a network server, the Corresponding Source may be on
a different server (operated by you or a third party) that supports
equivalent copying facilities, provided you maintain clear directions
next to the object code saying where to find the Corresponding Source.
Regardless of what server hosts the Corresponding Source, you remain
obligated to ensure that it is available for as long as needed to
satisfy these requirements.

   e) Convey the object code using peer-to-peer transmission, provided
you inform other peers where the object code and Corresponding Source of
the work are being offered to the general public at no charge under
subsection 6d.

   A separable portion of the object code, whose source code is excluded
from the Corresponding Source as a System Library, need not be included
in conveying the object code work.

   A "User Product" is either (1) a "consumer product", which means any
tangible personal property which is normally used for personal, family,
or household purposes, or (2) anything designed or sold for
incorporation into a dwelling.  In determining whether a product is a
consumer product, doubtful cases shall be resolved in favor of coverage.
For a particular product received by a particular user, "normally used"
refers to a typical or common use of that class of product, regardless
of the status of the particular user or of the way in which the
particular user actually uses, or expects or is expected to use, the
product.  A product is a consumer product regardless of whether the
product has substantial commercial, industrial or non-consumer uses,
unless such uses represent the only significant mode of use of the
product.

   "Installation Information" for a User Product means any methods,
procedures, authorization keys, or other information required to install
and execute modified versions of a covered work in that User Product
from a modified version of its Corresponding Source.  The information
must suffice to ensure that the continued functioning of the modified
object code is in no case prevented or interfered with solely because
modification has been made.

   If you convey an object code work under this section in, or with, or
specifically for use in, a User Product, and the conveying occurs as
part of a transaction in which the right of possession and use of the
User Product is transferred to the recipient in perpetuity or for a
fixed term (regardless of how the transaction is characterized), the
Corresponding Source conveyed under this section must be accompanied by
the Installation Information.  But this requirement does not apply if
neither you nor any third party retains the ability to install modified
object code on the User Product (for example, the work has been
installed in ROM).

   The requirement to provide Installation Information does not include
a requirement to continue to provide support service, warranty, or
updates for a work that has been modified or installed by the recipient,
or for the User Product in which it has been modified or installed.
Access to a network may be denied when the modification itself
materially and adversely affects the operation of the network or
violates the rules and protocols for communication across the network.

   Corresponding Source conveyed, and Installation Information provided,
in accord with this section must be in a format that is publicly
documented (and with an implementation available to the public in source
code form), and must require no special password or key for unpacking,
reading or copying.

   7.  Additional Terms.

   "Additional permissions" are terms that supplement the terms of this
License by making exceptions from one or more of its conditions.
Additional permissions that are applicable to the entire Program shall
be treated as though they were included in this License, to the extent
that they are valid under applicable law.  If additional permissions
apply only to part of the Program, that part may be used separately
under those permissions, but the entire Program remains governed by this
License without regard to the additional permissions.

   When you convey a copy of a covered work, you may at your option
remove any additional permissions from that copy, or from any part of
it.  (Additional permissions may be written to require their own removal
in certain cases when you modify the work.)  You may place additional
permissions on material, added by you to a covered work, for which you
have or can give appropriate copyright permission.

   Notwithstanding any other provision of this License, for material you
add to a covered work, you may (if authorized by the copyright holders
of that material) supplement the terms of this License with terms:

   a) Disclaiming warranty or limiting liability differently from the
terms of sections 15 and 16 of this License; or

   b) Requiring preservation of specified reasonable legal notices or
author attributions in that material or in the Appropriate Legal Notices
displayed by works containing it; or

   c) Prohibiting misrepresentation of the origin of that material, or
requiring that modified versions of such material be marked in
reasonable ways as different from the original version; or

   d) Limiting the use for publicity purposes of names of licensors or
authors of the material; or

   e) Declining to grant rights under trademark law for use of some
trade names, trademarks, or service marks; or

   f) Requiring indemnification of licensors and authors of that
material by anyone who conveys the material (or modified versions of it)
with contractual assumptions of liability to the recipient, for any
liability that these contractual assumptions directly impose on those
licensors and authors.

   All other non-permissive additional terms are considered "further
restrictions" within the meaning of section 10.  If the Program as you
received it, or any part of it, contains a notice stating that it is
governed by this License along with a term that is a further
restriction, you may remove that term.  If a license document contains a
further restriction but permits relicensing or conveying under this
License, you may add to a covered work material governed by the terms of
that license document, provided that the further restriction does not
survive such relicensing or conveying.

   If you add terms to a covered work in accord with this section, you
must place, in the relevant source files, a statement of the additional
terms that apply to those files, or a notice indicating where to find
the applicable terms.

   Additional terms, permissive or non-permissive, may be stated in the
form of a separately written license, or stated as exceptions; the above
requirements apply either way.

   8.  Termination.

   You may not propagate or modify a covered work except as expressly
provided under this License.  Any attempt otherwise to propagate or
modify it is void, and will automatically terminate your rights under
this License (including any patent licenses granted under the third
paragraph of section 11).

   However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the copyright
holder fails to notify you of the violation by some reasonable means
prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after your
receipt of the notice.

   Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License.  If your rights have been terminated and not permanently
reinstated, you do not qualify to receive new licenses for the same
material under section 10.

   9.  Acceptance Not Required for Having Copies.

   You are not required to accept this License in order to receive or
run a copy of the Program.  Ancillary propagation of a covered work
occurring solely as a consequence of using peer-to-peer transmission to
receive a copy likewise does not require acceptance.  However, nothing
other than this License grants you permission to propagate or modify any
covered work.  These actions infringe copyright if you do not accept
this License.  Therefore, by modifying or propagating a covered work,
you indicate your acceptance of this License to do so.

   10.  Automatic Licensing of Downstream Recipients.

   Each time you convey a covered work, the recipient automatically
receives a license from the original licensors, to run, modify and
propagate that work, subject to this License.  You are not responsible
for enforcing compliance by third parties with this License.

   An "entity transaction" is a transaction transferring control of an
organization, or substantially all assets of one, or subdividing an
organization, or merging organizations.  If propagation of a covered
work results from an entity transaction, each party to that transaction
who receives a copy of the work also receives whatever licenses to the
work the party’s predecessor in interest had or could give under the
previous paragraph, plus a right to possession of the Corresponding
Source of the work from the predecessor in interest, if the predecessor
has it or can get it with reasonable efforts.

   You may not impose any further restrictions on the exercise of the
rights granted or affirmed under this License.  For example, you may not
impose a license fee, royalty, or other charge for exercise of rights
granted under this License, and you may not initiate litigation
(including a cross-claim or counterclaim in a lawsuit) alleging that any
patent claim is infringed by making, using, selling, offering for sale,
or importing the Program or any portion of it.

   11.  Patents.

   A "contributor" is a copyright holder who authorizes use under this
License of the Program or a work on which the Program is based.  The
work thus licensed is called the contributor’s "contributor version".

   A contributor’s "essential patent claims" are all patent claims owned
or controlled by the contributor, whether already acquired or hereafter
acquired, that would be infringed by some manner, permitted by this
License, of making, using, or selling its contributor version, but do
not include claims that would be infringed only as a consequence of
further modification of the contributor version.  For purposes of this
definition, "control" includes the right to grant patent sublicenses in
a manner consistent with the requirements of this License.

   Each contributor grants you a non-exclusive, worldwide, royalty-free
patent license under the contributor’s essential patent claims, to make,
use, sell, offer for sale, import and otherwise run, modify and
propagate the contents of its contributor version.

   In the following three paragraphs, a "patent license" is any express
agreement or commitment, however denominated, not to enforce a patent
(such as an express permission to practice a patent or covenant not to
sue for patent infringement).  To "grant" such a patent license to a
party means to make such an agreement or commitment not to enforce a
patent against the party.

   If you convey a covered work, knowingly relying on a patent license,
and the Corresponding Source of the work is not available for anyone to
copy, free of charge and under the terms of this License, through a
publicly available network server or other readily accessible means,
then you must either (1) cause the Corresponding Source to be so
available, or (2) arrange to deprive yourself of the benefit of the
patent license for this particular work, or (3) arrange, in a manner
consistent with the requirements of this License, to extend the patent
license to downstream recipients.  "Knowingly relying" means you have
actual knowledge that, but for the patent license, your conveying the
covered work in a country, or your recipient’s use of the covered work
in a country, would infringe one or more identifiable patents in that
country that you have reason to believe are valid.

   If, pursuant to or in connection with a single transaction or
arrangement, you convey, or propagate by procuring conveyance of, a
covered work, and grant a patent license to some of the parties
receiving the covered work authorizing them to use, propagate, modify or
convey a specific copy of the covered work, then the patent license you
grant is automatically extended to all recipients of the covered work
and works based on it.

   A patent license is "discriminatory" if it does not include within
the scope of its coverage, prohibits the exercise of, or is conditioned
on the non-exercise of one or more of the rights that are specifically
granted under this License.  You may not convey a covered work if you
are a party to an arrangement with a third party that is in the business
of distributing software, under which you make payment to the third
party based on the extent of your activity of conveying the work, and
under which the third party grants, to any of the parties who would
receive the covered work from you, a discriminatory patent license (a)
in connection with copies of the covered work conveyed by you (or copies
made from those copies), or (b) primarily for and in connection with
specific products or compilations that contain the covered work, unless
you entered into that arrangement, or that patent license was granted,
prior to 28 March 2007.

   Nothing in this License shall be construed as excluding or limiting
any implied license or other defenses to infringement that may otherwise
be available to you under applicable patent law.

   12.  No Surrender of Others’ Freedom.

   If conditions are imposed on you (whether by court order, agreement
or otherwise) that contradict the conditions of this License, they do
not excuse you from the conditions of this License.  If you cannot
convey a covered work so as to satisfy simultaneously your obligations
under this License and any other pertinent obligations, then as a
consequence you may not convey it at all.  For example, if you agree to
terms that obligate you to collect a royalty for further conveying from
those to whom you convey the Program, the only way you could satisfy
both those terms and this License would be to refrain entirely from
conveying the Program.

   13.  Use with the GNU Affero General Public License.

   Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU Affero General Public License into a single
combined work, and to convey the resulting work.  The terms of this
License will continue to apply to the part which is the covered work,
but the special requirements of the GNU Affero General Public License,
section 13, concerning interaction through a network will apply to the
combination as such.

   14.  Revised Versions of this License.

   The Free Software Foundation may publish revised and/or new versions
of the GNU General Public License from time to time.  Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns.

   Each version is given a distinguishing version number.  If the
Program specifies that a certain numbered version of the GNU General
Public License "or any later version" applies to it, you have the option
of following the terms and conditions either of that numbered version or
of any later version published by the Free Software Foundation.  If the
Program does not specify a version number of the GNU General Public
License, you may choose any version ever published by the Free Software
Foundation.

   If the Program specifies that a proxy can decide which future
versions of the GNU General Public License can be used, that proxy’s
public statement of acceptance of a version permanently authorizes you
to choose that version for the Program.

   Later license versions may give you additional or different
permissions.  However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.

   15.  Disclaimer of Warranty.

   THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT
WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF
THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME
THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

   16.  Limitation of Liability.

   IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR
CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES
ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT
NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES
SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE
WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

   17.  Interpretation of Sections 15 and 16.

   If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely approximates an
absolute waiver of all civil liability in connection with the Program,
unless a warranty or assumption of liability accompanies a copy of the
Program in return for a fee.

   END OF TERMS AND CONDITIONS

   How to Apply These Terms to Your New Programs

   If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.

   <one line to give the program’s name and a brief idea of what it
does.> Copyright (C) <year> <name of author>

   This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.

   This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.

   You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

   Also add information on how to contact you by electronic and paper
mail.

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

   <program> Copyright (C) <year> <name of author> This program comes
with ABSOLUTELY NO WARRANTY; for details type ‘show w’.  This is free
software, and you are welcome to redistribute it under certain
conditions; type ‘show c’ for details.

   The hypothetical commands ‘show w’ and ‘show c’ should show the
appropriate parts of the General Public License.  Of course, your
program’s commands might be different; for a GUI interface, you would
use an "about box".

   You should also get your employer (if you work as a programmer) or
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  For more information on this, and how to apply and follow
the GNU GPL, see <http://www.gnu.org/licenses/>.

   The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
GNU Lesser General Public License instead of this License.  But first,
please read <http://www.gnu.org/philosophy/why-not-lgpl.html>.



Tag Table:
Node: Top383
Node: Chapter 11408
Node: Chapter 217057
Node: Chapter 321468
Node: Section 3.122402
Node: Section 3.233305
Node: Section 3.337691
Node: Section 3.440681
Node: Section 3.541484
Node: Section 3.642097
Node: Section 3.743384
Node: Section 3.845315
Node: Section 3.945488
Node: Section 3.1048738
Node: Section 3.1149316
Node: Section 3.1250712
Node: Section 3.1354667
Node: Section 3.1455431
Node: Section 3.1557420
Node: Section 3.1667331
Node: Section 3.1769116
Node: Section 3.1872220
Node: Chapter 474826
Node: Chapter 576574
Node: Chapter 677703
Node: Section 6.178122
Node: Section 6.278809
Node: Chapter 779640
Node: Section 7.180143
Node: Section 7.2105452

End Tag Table


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