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# gb_sets (stdlib v7.0)

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Sets represented by general balanced trees.

This module provides ordered sets using Prof. Arne Andersson's General Balanced Trees. Ordered sets can be much more efficient than using ordered lists, for larger sets, but depends on the application.

The data representing a set as used by this module is to be regarded as opaque by other modules. In abstract terms, the representation is a composite type of existing Erlang terms. See note on data types. Any code assuming knowledge of the format is running on thin ice.

This module considers two elements as different if and only if they do not compare equal (`==`).

## Complexity Note

The complexity on set operations is bounded by either O(|S|) or O(|T| log(|S|))*, where S is the largest given set, depending on which is fastest for any particular function call. For operating on sets of almost equal size, this implementation is about 3 times slower than using ordered-list sets directly. For sets of very different sizes, however, this solution can be arbitrarily much faster; in practical cases, often 10-100 times. This implementation is particularly suited for accumulating elements a few at a time, building up a large set (> 100-200 elements), and repeatedly testing for membership in the current set.

As with normal tree structures, lookup (membership testing), insertion, and deletion have logarithmic complexity.

## Compatibility

See the Compatibility Section in the `sets` module for information about the compatibility of the different implementations of sets in the Standard Library.

## See Also

`gb_trees`, `ordsets`, `sets`

# Summary

## Types

iter()
iter(Element)

A general balanced set iterator.

set()
set(Element)

A general balanced set.

## Functions

add(Element, Set1)

Equivalent to `add_element(Element, Set1)`.

add_element(Element, Set1)

Returns a new set formed from `Set1` with `Element` inserted.

balance(Set1)

Rebalances the tree representation of `Set1`.

del_element(Element, Set1)

Equivalent to `delete_any(Element, Set1)`.

delete(Element, Set1)

Returns a new set formed from `Set1` with `Element` removed, assuming `Element` is present in `Set1`.

delete_any(Element, Set1)

Returns a new set formed from `Set1` with `Element` removed.

difference(Set1, Set2)

Equivalent to `subtract(Set1, Set2)`.

empty()

Returns a new empty set.

filter(Pred, Set1)

Filters elements in `Set1` using predicate function `Pred`.

filtermap(Fun, Set1)

Calls `Fun(Elem)` for each `Elem` of `Set1` to update or remove elements from `Set1`.

fold(Function, Acc0, Set)

Folds `Function` over every element in `Set` and returns the final value of the accumulator.

from_list(List)

Returns a set of the elements in `List`, where `List` can be unordered and contain duplicates.

from_ordset(List)

Turns an ordered list without duplicates `List` into a set.

insert(Element, Set1)

Returns a new set formed from `Set1` with `Element` inserted, assuming `Element` is not already present.

intersection(SetList)

Returns the intersection of the non-empty list of sets.

intersection(Set1, Set2)

Returns the intersection of `Set1` and `Set2`.

is_disjoint(Set1, Set2)

Returns `true` if `Set1` and `Set2` are disjoint; otherwise, returns `false`.

is_element(Element, Set)

Equivalent to `is_member(Element, Set)`.

is_empty(Set)

Returns `true` if `Set` is an empty set; otherwise, returns `false`.

is_equal(Set1, Set2)

Returns `true` if `Set1` and `Set2` are equal, that is, if every element of one set is also a member of the other set; otherwise, returns `false`.

is_member(Element, Set)

Returns `true` if `Element` is an element of `Set`; otherwise, returns `false`.

is_set(Term)

Returns `true` if `Term` appears to be a set; otherwise, returns `false`.

is_subset(Set1, Set2)

Returns `true` when every element of `Set1` is also a member of `Set2`; otherwise, returns `false`.

iterator(Set)

Returns an iterator that can be used for traversing the entries of `Set`; see `next/1`.

iterator(Set, Order)

Returns an iterator that can be used for traversing the entries of `Set` in either `ordered` or `reversed` direction; see `next/1`.

iterator_from(Element, Set)

Returns an iterator that can be used for traversing the entries of `Set`; see `next/1`.

iterator_from(Element, Set, Order)

Returns an iterator that can be used for traversing the entries of `Set`; see `next/1`.

larger(Element1, Set)

Returns `{found, Element2}`, where `Element2` is the least element strictly greater than `Element1`.

largest(Set)

Returns the largest element in `Set`.

map(Fun, Set1)

Maps elements in `Set1` with mapping function `Fun`.

new()

Returns a new empty set.

next(Iter1)

Returns `{Element, Iter2}`, where `Element` is the first element referred to by iterator `Iter1`, and `Iter2` is the new iterator to be used for traversing the remaining elements, or the atom `none` if no elements remain.

singleton(Element)

Returns a set containing only element `Element`.

size(Set)

Returns the number of elements in `Set`.

smaller(Element1, Set)

Returns `{found, Element2}`, where `Element2` is the greatest element strictly less than `Element1`.

smallest(Set)

Returns the smallest element in `Set`.

subtract(Set1, Set2)

Returns the elements of `Set1` that are not elements in `Set2`.

take_largest(Set1)

Returns `{Element, Set2}`, where `Element` is the largest element in `Set1`, and `Set2` is this set with `Element` deleted.

take_smallest(Set1)

Returns `{Element, Set2}`, where `Element` is the smallest element in `Set1`, and `Set2` is this set with `Element` deleted.

to_list(Set)

Returns the elements of `Set` as an ordered list.

union(SetList)

Returns the union of a list of sets.

union(Set1, Set2)

Returns the union of `Set1` and `Set2`.

# Types

# iter()

    -type iter() :: iter(_).

# iter(Element)

    -opaque iter(Element)

A general balanced set iterator.

# set()

    -type set() :: set(_).

# set(Element)

    -opaque set(Element)

A general balanced set.

# Functions

# add(Element, Set1)

    -spec add(Element, Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Equivalent to `add_element(Element, Set1)`.

# add_element(Element, Set1)

    -spec add_element(Element, Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Returns a new set formed from `Set1` with `Element` inserted.

If `Element` is already an element in `Set1`, nothing is changed.

## Examples

    1> S0 = gb_sets:new().
    2> S1 = gb_sets:add_element(7, S0).
    3> gb_sets:to_list(S1).
    [7]
    4> S2 = gb_sets:add_element(42, S1).
    5> S2 = gb_sets:add_element(42, S1).
    6> gb_sets:to_list(S2).
    [7,42]

# balance(Set1)

    -spec balance(Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Rebalances the tree representation of `Set1`.

This is rarely necessary, but can be motivated when a large number of elements have been deleted from the tree without further insertions. Forcing rebalancing can minimize lookup times, as deletion does not rebalance the tree.

## Examples

    1> S0 = gb_sets:from_ordset(lists:seq(1, 100)).
    2> Delete = fun(E, Set) -> gb_sets:delete(E, Set) end.
    3> S1 = lists:foldl(Delete, S0, lists:seq(1, 50)).
    4> gb_sets:size(S1).
    50
    5> S2 = gb_sets:balance(S1).

# del_element(Element, Set1)

    -spec del_element(Element, Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Equivalent to `delete_any(Element, Set1)`.

# delete(Element, Set1)

    -spec delete(Element, Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Returns a new set formed from `Set1` with `Element` removed, assuming `Element` is present in `Set1`.

Use `delete_any/2` when deleting from a set where `Element` is potentially missing.

## Examples

    1> S = gb_sets:from_list([a,b]).
    2> gb_sets:to_list(gb_sets:delete(b, S)).
    [a]

# delete_any(Element, Set1)

    -spec delete_any(Element, Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Returns a new set formed from `Set1` with `Element` removed.

If `Element` is not an element in `Set1`, nothing is changed.

## Examples

    1> S = gb_sets:from_list([a,b]).
    2> gb_sets:to_list(gb_sets:delete_any(b, S)).
    [a]
    3> S = gb_sets:delete_any(x, S).

# difference(Set1, Set2)

    -spec difference(Set1, Set2) -> Set3
                        when Set1 :: set(Element), Set2 :: set(Element), Set3 :: set(Element).

Equivalent to `subtract(Set1, Set2)`.

# empty()

    -spec empty() -> Set when Set :: set(none()).

Returns a new empty set.

## Examples

    1> gb_sets:to_list(gb_sets:empty()).
    []

# filter(Pred, Set1)

    -spec filter(Pred, Set1) -> Set2
                    when Pred :: fun((Element) -> boolean()), Set1 :: set(Element), Set2 :: set(Element).

Filters elements in `Set1` using predicate function `Pred`.

## Examples

    1> S = gb_sets:from_list([1,2,3,4,5,6,7]).
    2> IsEven = fun(N) -> N rem 2 =:= 0 end.
    3> Filtered = gb_sets:filter(IsEven, S).
    4> gb_sets:to_list(Filtered).
    [2,4,6]

# filtermap(Fun, Set1)

(since OTP 27.0)

    -spec filtermap(Fun, Set1) -> Set2
                       when
                           Fun :: fun((Element1) -> boolean() | {true, Element2}),
                           Set1 :: set(Element1),
                           Set2 :: set(Element1 | Element2).

Calls `Fun(Elem)` for each `Elem` of `Set1` to update or remove elements from `Set1`.

`Fun/1` must return either a Boolean or a tuple `{true, Value}`. The function returns the set of elements for which `Fun` returns a new value, with `true` being equivalent to `{true, Elem}`.

`gb_sets:filtermap/2` behaves as if it were defined as follows:

    filtermap(Fun, Set1) ->
        gb_sets:from_list(lists:filtermap(Fun, Set1)).

## Examples

    1> S = gb_sets:from_list([2,4,5,6,8,9])
    2> F = fun(X) ->
               case X rem 2 of
                   0 -> {true, X div 2};
                   1 -> false
               end
            end.
    3> Set = gb_sets:filtermap(F, S).
    4> gb_sets:to_list(Set).
    [1,2,3,4]

# fold(Function, Acc0, Set)

    -spec fold(Function, Acc0, Set) -> Acc1
                  when
                      Function :: fun((Element, AccIn) -> AccOut),
                      Acc0 :: Acc,
                      Acc1 :: Acc,
                      AccIn :: Acc,
                      AccOut :: Acc,
                      Set :: set(Element).

Folds `Function` over every element in `Set` and returns the final value of the accumulator.

## Examples

    1> S = gb_sets:from_list([1,2,3,4]).
    2> Plus = fun erlang:'+'/2.
    3> gb_sets:fold(Plus, 0, S).
    10

# from_list(List)

    -spec from_list(List) -> Set when List :: [Element], Set :: set(Element).

Returns a set of the elements in `List`, where `List` can be unordered and contain duplicates.

## Examples

    1> Unordered = [x,y,a,x,y,b,b,z]
    2> gb_sets:to_list(gb_sets:from_list(Unordered)).
    [a,b,x,y,z]

# from_ordset(List)

    -spec from_ordset(List) -> Set when List :: [Element], Set :: set(Element).

Turns an ordered list without duplicates `List` into a set.

See `from_list/1` for a function that accepts unordered lists with duplicates.

## Examples

    1> Ordset = [1,2,3].
    2> gb_sets:to_list(gb_sets:from_ordset(Ordset)).
    [1,2,3]

# insert(Element, Set1)

    -spec insert(Element, Set1) -> Set2 when Set1 :: set(Element), Set2 :: set(Element).

Returns a new set formed from `Set1` with `Element` inserted, assuming `Element` is not already present.

Use `add/2` for inserting into a set where `Element` is potentially already present.

## Examples

    1> S0 = gb_sets:new().
    2> S1 = gb_sets:insert(7, S0).
    3> gb_sets:to_list(S1).
    [7]
    4> S2 = gb_sets:insert(42, S1).
    5> gb_sets:to_list(S2).
    [7,42]

# intersection(SetList)

    -spec intersection(SetList) -> Set when SetList :: [set(Element), ...], Set :: set(Element).

Returns the intersection of the non-empty list of sets.

The intersection of multiple sets is a new set that contains only the elements that are present in all sets.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([d,e,f]).
    3> S2 = gb_sets:from_list([q,r])
    4> Sets = [S0, S1, S2].
    5> gb_sets:to_list(gb_sets:intersection([S0, S1, S2])).
    []
    6> gb_sets:to_list(gb_sets:intersection([S0, S1])).
    [d]
    7> gb_sets:intersection([]).
    ** exception error: no function clause matching gb_sets:intersection([])

# intersection(Set1, Set2)

    -spec intersection(Set1, Set2) -> Set3
                          when Set1 :: set(Element), Set2 :: set(Element), Set3 :: set(Element).

Returns the intersection of `Set1` and `Set2`.

The intersection of two sets is a new set that contains only the elements that are present in both sets.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([c,d,e,f]).
    3> S2 = gb_sets:from_list([q,r]).
    4> gb_sets:to_list(gb_sets:intersection(S0, S1)).
    [c,d]
    5> gb_sets:to_list(gb_sets:intersection(S1, S2)).
    []

# is_disjoint(Set1, Set2)

    -spec is_disjoint(Set1, Set2) -> boolean() when Set1 :: set(Element), Set2 :: set(Element).

Returns `true` if `Set1` and `Set2` are disjoint; otherwise, returns `false`.

Two sets are disjoint if they have no elements in common.

This function is equivalent to `gb_sets:intersection(Set1, Set2) =:= []`, but faster.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([d,e,f]).
    3> S2 = gb_sets:from_list([q,r])
    4> gb_sets:is_disjoint(S0, S1).
    false
    5> gb_sets:is_disjoint(S1, S2).
    true

# is_element(Element, Set)

    -spec is_element(Element, Set) -> boolean() when Set :: set(Element).

Equivalent to `is_member(Element, Set)`.

# is_empty(Set)

    -spec is_empty(Set) -> boolean() when Set :: set().

Returns `true` if `Set` is an empty set; otherwise, returns `false`.

## Examples

    1> gb_sets:is_empty(gb_sets:new()).
    true
    2> gb_sets:is_empty(gb_sets:singleton(1)).
    false

# is_equal(Set1, Set2)

(since OTP 27.0)

    -spec is_equal(Set1, Set2) -> boolean() when Set1 :: set(), Set2 :: set().

Returns `true` if `Set1` and `Set2` are equal, that is, if every element of one set is also a member of the other set; otherwise, returns `false`.

## Examples

    1> Empty = gb_sets:new().
    2> S = gb_sets:from_list([a,b]).
    3> gb_sets:is_equal(S, S)
    true
    4> gb_sets:is_equal(S, Empty)
    false

# is_member(Element, Set)

    -spec is_member(Element, Set) -> boolean() when Set :: set(Element).

Returns `true` if `Element` is an element of `Set`; otherwise, returns `false`.

## Examples

    1> S = gb_sets:from_list([a,b,c]).
    2> gb_sets:is_member(42, S).
    false
    3> gb_sets:is_member(b, S).
    true

# is_set(Term)

    -spec is_set(Term) -> boolean() when Term :: term().

Returns `true` if `Term` appears to be a set; otherwise, returns `false`.

#### Note

This function will return `true` for any term that coincides with the representation of a `gb_set`, while not really being a `gb_set`, thus it might return false positive results. See also note on data types.

Furthermore, since gb_sets are opaque, calling this function on terms that are not gb_sets could result in `dialyzer` warnings.

## Examples

    1> gb_sets:is_set(gb_sets:new()).
    true
    2> gb_sets:is_set(gb_sets:singleton(42)).
    true
    3> gb_sets:is_set(0).
    false

# is_subset(Set1, Set2)

    -spec is_subset(Set1, Set2) -> boolean() when Set1 :: set(Element), Set2 :: set(Element).

Returns `true` when every element of `Set1` is also a member of `Set2`; otherwise, returns `false`.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([c,d]).
    3> gb_sets:is_subset(S1, S0).
    true
    4> gb_sets:is_subset(S0, S1).
    false
    5> gb_sets:is_subset(S0, S0).
    true

# iterator(Set)

    -spec iterator(Set) -> Iter when Set :: set(Element), Iter :: iter(Element).

Returns an iterator that can be used for traversing the entries of `Set`; see `next/1`.

Equivalent to `iterator(Set, ordered)`.

# iterator(Set, Order)

(since OTP 27.0)

    -spec iterator(Set, Order) -> Iter
                      when Set :: set(Element), Iter :: iter(Element), Order :: ordered | reversed.

Returns an iterator that can be used for traversing the entries of `Set` in either `ordered` or `reversed` direction; see `next/1`.

The implementation is very efficient; traversing the whole set using `next/1` is only slightly slower than getting the list of all elements using `to_list/1` and traversing that. The main advantage of the iterator approach is that it avoids building the complete list of all elements to be built in memory at once.

    1> S = gb_sets:from_ordset([1,2,3,4,5]).
    2> Iter0 = gb_sets:iterator(S, ordered).
    3> element(1, gb_sets:next(Iter0)).
    1
    4> Iter1 = gb_sets:iterator(S, reversed).
    5> element(1, gb_sets:next(Iter1)).
    5

# iterator_from(Element, Set)

(since OTP 18.0)

    -spec iterator_from(Element, Set) -> Iter when Set :: set(Element), Iter :: iter(Element).

Returns an iterator that can be used for traversing the entries of `Set`; see `next/1`.

Unlike the iterator returned by `iterator/1` or `iterator/2`, this iterator starts with the first element greater than or equal to `Element`.

Equivalent to `iterator_from(Element, Set, ordered)`.

## Examples

    1> S = gb_sets:from_ordset([10,20,30,40,50]).
    2> Iter = gb_sets:iterator_from(17, S).
    3> element(1, gb_sets:next(Iter)).
    20

# iterator_from(Element, Set, Order)

(since OTP 27.0)

    -spec iterator_from(Element, Set, Order) -> Iter
                           when Set :: set(Element), Iter :: iter(Element), Order :: ordered | reversed.

Returns an iterator that can be used for traversing the entries of `Set`; see `next/1`.

Unlike the iterator returned by `iterator/1` or `iterator/2`, this iterator starts with the first element greater than or equal to `Element`.

## Examples

    1> S = gb_sets:from_ordset([10,20,30,40,50]).
    2> Iter = gb_sets:iterator_from(17, S, reversed).
    3> element(1, gb_sets:next(Iter)).
    10

# larger(Element1, Set)

(since OTP 27.0)

    -spec larger(Element1, Set) -> none | {found, Element2}
                    when Element1 :: Element, Element2 :: Element, Set :: set(Element).

Returns `{found, Element2}`, where `Element2` is the least element strictly greater than `Element1`.

Returns `none` if no such element exists.

## Examples

    1> S = gb_sets:from_list([10,20,30]).
    2> gb_sets:larger(1, S).
    {found,10}
    3> gb_sets:larger(10, S).
    {found,20}
    4> gb_sets:larger(19, S).
    {found,20}
    5> gb_sets:larger(30, S).
    none

# largest(Set)

    -spec largest(Set) -> Element when Set :: set(Element).

Returns the largest element in `Set`.

Assumes that `Set` is not empty.

## Examples

    1> S = gb_sets:from_list([a,b,c]).
    2> gb_sets:largest(S).
    c

# map(Fun, Set1)

(since OTP 27.0)

    -spec map(Fun, Set1) -> Set2
                 when Fun :: fun((Element1) -> Element2), Set1 :: set(Element1), Set2 :: set(Element2).

Maps elements in `Set1` with mapping function `Fun`.

## Examples

    1> S = gb_sets:from_list([1,2,3,4,5,6,7]).
    2> F = fun(N) -> N div 2 end.
    3> Mapped = gb_sets:map(F, S).
    4> gb_sets:to_list(Mapped).
    [0,1,2,3]

# new()

    -spec new() -> Set when Set :: set(none()).

Returns a new empty set.

## Examples

    1> gb_sets:to_list(gb_sets:new()).
    []

# next(Iter1)

    -spec next(Iter1) -> {Element, Iter2} | none when Iter1 :: iter(Element), Iter2 :: iter(Element).

Returns `{Element, Iter2}`, where `Element` is the first element referred to by iterator `Iter1`, and `Iter2` is the new iterator to be used for traversing the remaining elements, or the atom `none` if no elements remain.

    1> S = gb_sets:from_ordset([1,2,3,4,5]).
    2> Iter0 = gb_sets:iterator(S).
    3> {Element0, Iter1} = gb_sets:next(Iter0).
    4> Element0.
    1
    5> {Element1, Iter2} = gb_sets:next(Iter1).
    6> Element1.
    2

# singleton(Element)

    -spec singleton(Element) -> set(Element).

Returns a set containing only element `Element`.

## Examples

    1> S = gb_sets:singleton(42).
    2> gb_sets:to_list(S).
    [42]

# size(Set)

    -spec size(Set) -> non_neg_integer() when Set :: set().

Returns the number of elements in `Set`.

## Examples

    1> gb_sets:size(gb_sets:new()).
    0
    2> gb_sets:size(gb_sets:from_list([4,5,6])).
    3

# smaller(Element1, Set)

(since OTP 27.0)

    -spec smaller(Element1, Set) -> none | {found, Element2}
                     when Element1 :: Element, Element2 :: Element, Set :: set(Element).

Returns `{found, Element2}`, where `Element2` is the greatest element strictly less than `Element1`.

Returns `none` if no such element exists.

## Examples

    1> S = gb_sets:from_list([a,b,c]).
    2> gb_sets:smaller(b, S).
    {found,a}
    3> gb_sets:smaller(z, S).
    {found,c}
    4> gb_sets:smaller(a, S).
    none

# smallest(Set)

    -spec smallest(Set) -> Element when Set :: set(Element).

Returns the smallest element in `Set`.

Assumes that `Set` is not empty.

## Examples

    1> S = gb_sets:from_list([a,b,c]).
    2> gb_sets:smallest(S).
    a

# subtract(Set1, Set2)

    -spec subtract(Set1, Set2) -> Set3 when Set1 :: set(Element), Set2 :: set(Element), Set3 :: set(Element).

Returns the elements of `Set1` that are not elements in `Set2`.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([c,d,e,f]).
    3> gb_sets:to_list(gb_sets:subtract(S0, S1)).
    [a,b]
    4> gb_sets:to_list(gb_sets:subtract(S1, S0)).
    [e,f]

# take_largest(Set1)

    -spec take_largest(Set1) -> {Element, Set2} when Set1 :: set(Element), Set2 :: set(Element).

Returns `{Element, Set2}`, where `Element` is the largest element in `Set1`, and `Set2` is this set with `Element` deleted.

Assumes that `Set1` is not empty.

## Examples

    1> S0 = gb_sets:from_list([a,b,c]).
    2> {Largest,S1} = gb_sets:take_largest(S0).
    3> Largest.
    c
    4> gb_sets:to_list(S1).
    [a,b]

# take_smallest(Set1)

    -spec take_smallest(Set1) -> {Element, Set2} when Set1 :: set(Element), Set2 :: set(Element).

Returns `{Element, Set2}`, where `Element` is the smallest element in `Set1`, and `Set2` is this set with `Element` deleted.

Assumes that `Set1` is not empty.

## Examples

    1> S0 = gb_sets:from_list([a,b,c]).
    2> {Smallest,S1} = gb_sets:take_smallest(S0).
    3> Smallest.
    a
    4> gb_sets:to_list(S1).
    [b,c]

# to_list(Set)

    -spec to_list(Set) -> List when Set :: set(Element), List :: [Element].

Returns the elements of `Set` as an ordered list.

    1> gb_sets:to_list(gb_sets:from_list([4,3,5,1,2])).
    [1,2,3,4,5]

# union(SetList)

    -spec union(SetList) -> Set when SetList :: [set(Element), ...], Set :: set(Element).

Returns the union of a list of sets.

The union of multiple sets is a new set that contains all the elements from all sets, without duplicates.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([d,e,f]).
    3> S2 = gb_sets:from_list([q,r])
    4> Sets = [S0, S1, S2].
    5> Union = gb_sets:union(Sets).
    6> gb_sets:to_list(Union).
    [a,b,c,d,e,f,q,r]

# union(Set1, Set2)

    -spec union(Set1, Set2) -> Set3 when Set1 :: set(Element), Set2 :: set(Element), Set3 :: set(Element).

Returns the union of `Set1` and `Set2`.

The union of two sets is a new set that contains all the elements from both sets, without duplicates.

## Examples

    1> S0 = gb_sets:from_list([a,b,c,d]).
    2> S1 = gb_sets:from_list([c,d,e,f]).
    3> Union = gb_sets:union(S0, S1).
    4> gb_sets:to_list(Union).
    [a,b,c,d,e,f]