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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    // This is pretty much entirely stolen from TreeSet, since BTreeMap has an identical interface
    // to TreeMap

    use core::cmp::Ordering::{self, Less, Greater, Equal};
    use core::cmp::{min, max};
    use core::fmt::Debug;
    use core::fmt;
    use core::iter::{Peekable, FromIterator, FusedIterator};
    use core::ops::{BitOr, BitAnd, BitXor, Sub, RangeBounds};

    use borrow::Borrow;
    use collections::btree_map::{self, BTreeMap, Keys};
    use super::Recover;

    // FIXME(conventions): implement bounded iterators

    /// A set based on a B-Tree.
    ///
    /// See [`BTreeMap`]'s documentation for a detailed discussion of this collection's performance
    /// benefits and drawbacks.
    ///
    /// It is a logic error for an item to be modified in such a way that the item's ordering relative
    /// to any other item, as determined by the [`Ord`] trait, changes while it is in the set. This is
    /// normally only possible through [`Cell`], [`RefCell`], global state, I/O, or unsafe code.
    ///
    /// [`BTreeMap`]: struct.BTreeMap.html
    /// [`Ord`]: ../../std/cmp/trait.Ord.html
    /// [`Cell`]: ../../std/cell/struct.Cell.html
    /// [`RefCell`]: ../../std/cell/struct.RefCell.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::BTreeSet;
    ///
    /// // Type inference lets us omit an explicit type signature (which
    /// // would be `BTreeSet<&str>` in this example).
    /// let mut books = BTreeSet::new();
    ///
    /// // Add some books.
    /// books.insert("A Dance With Dragons");
    /// books.insert("To Kill a Mockingbird");
    /// books.insert("The Odyssey");
    /// books.insert("The Great Gatsby");
    ///
    /// // Check for a specific one.
    /// if !books.contains("The Winds of Winter") {
    ///     println!("We have {} books, but The Winds of Winter ain't one.",
    ///              books.len());
    /// }
    ///
    /// // Remove a book.
    /// books.remove("The Odyssey");
    ///
    /// // Iterate over everything.
    /// for book in &books {
    ///     println!("{}", book);
    /// }
    /// ```
    #[derive(Clone, Hash, PartialEq, Eq, Ord, PartialOrd)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BTreeSet<T> {
        map: BTreeMap<T, ()>,
    }

    /// An iterator over the items of a `BTreeSet`.
    ///
    /// This `struct` is created by the [`iter`] method on [`BTreeSet`].
    /// See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`iter`]: struct.BTreeSet.html#method.iter
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> {
        iter: Keys<'a, T, ()>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Iter<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Iter")
             .field(&self.iter.clone())
             .finish()
        }
    }

    /// An owning iterator over the items of a `BTreeSet`.
    ///
    /// This `struct` is created by the [`into_iter`] method on [`BTreeSet`][`BTreeSet`]
    /// (provided by the `IntoIterator` trait). See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`into_iter`]: struct.BTreeSet.html#method.into_iter
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct IntoIter<T> {
        iter: btree_map::IntoIter<T, ()>,
    }

    /// An iterator over a sub-range of items in a `BTreeSet`.
    ///
    /// This `struct` is created by the [`range`] method on [`BTreeSet`].
    /// See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`range`]: struct.BTreeSet.html#method.range
    #[derive(Debug)]
    #[stable(feature = "btree_range", since = "1.17.0")]
    pub struct Range<'a, T: 'a> {
        iter: btree_map::Range<'a, T, ()>,
    }

    /// A lazy iterator producing elements in the difference of `BTreeSet`s.
    ///
    /// This `struct` is created by the [`difference`] method on [`BTreeSet`].
    /// See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`difference`]: struct.BTreeSet.html#method.difference
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Difference<'a, T: 'a> {
        a: Peekable<Iter<'a, T>>,
        b: Peekable<Iter<'a, T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Difference<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Difference")
             .field(&self.a)
             .field(&self.b)
             .finish()
        }
    }

    /// A lazy iterator producing elements in the symmetric difference of `BTreeSet`s.
    ///
    /// This `struct` is created by the [`symmetric_difference`] method on
    /// [`BTreeSet`]. See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`symmetric_difference`]: struct.BTreeSet.html#method.symmetric_difference
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct SymmetricDifference<'a, T: 'a> {
        a: Peekable<Iter<'a, T>>,
        b: Peekable<Iter<'a, T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for SymmetricDifference<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("SymmetricDifference")
             .field(&self.a)
             .field(&self.b)
             .finish()
        }
    }

    /// A lazy iterator producing elements in the intersection of `BTreeSet`s.
    ///
    /// This `struct` is created by the [`intersection`] method on [`BTreeSet`].
    /// See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`intersection`]: struct.BTreeSet.html#method.intersection
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Intersection<'a, T: 'a> {
        a: Peekable<Iter<'a, T>>,
        b: Peekable<Iter<'a, T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Intersection<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Intersection")
             .field(&self.a)
             .field(&self.b)
             .finish()
        }
    }

    /// A lazy iterator producing elements in the union of `BTreeSet`s.
    ///
    /// This `struct` is created by the [`union`] method on [`BTreeSet`].
    /// See its documentation for more.
    ///
    /// [`BTreeSet`]: struct.BTreeSet.html
    /// [`union`]: struct.BTreeSet.html#method.union
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Union<'a, T: 'a> {
        a: Peekable<Iter<'a, T>>,
        b: Peekable<Iter<'a, T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Union<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Union")
             .field(&self.a)
             .field(&self.b)
             .finish()
        }
    }

    impl<T: Ord> BTreeSet<T> {
        /// Makes a new `BTreeSet` with a reasonable choice of B.
        ///
        /// # Examples
        ///
        /// ```
        /// # #![allow(unused_mut)]
        /// use std::collections::BTreeSet;
        ///
        /// let mut set: BTreeSet<i32> = BTreeSet::new();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> BTreeSet<T> {
            BTreeSet { map: BTreeMap::new() }
        }

        /// Constructs a double-ended iterator over a sub-range of elements in the set.
        /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
        /// yield elements from min (inclusive) to max (exclusive).
        /// The range may also be entered as `(Bound<T>, Bound<T>)`, so for example
        /// `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive
        /// range from 4 to 10.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        /// use std::ops::Bound::Included;
        ///
        /// let mut set = BTreeSet::new();
        /// set.insert(3);
        /// set.insert(5);
        /// set.insert(8);
        /// for &elem in set.range((Included(&4), Included(&8))) {
        ///     println!("{}", elem);
        /// }
        /// assert_eq!(Some(&5), set.range(4..).next());
        /// ```
        #[stable(feature = "btree_range", since = "1.17.0")]
        pub fn range<K: ?Sized, R>(&self, range: R) -> Range<T>
            where K: Ord, T: Borrow<K>, R: RangeBounds<K>
        {
            Range { iter: self.map.range(range) }
        }

        /// Visits the values representing the difference,
        /// i.e. the values that are in `self` but not in `other`,
        /// in ascending order.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut a = BTreeSet::new();
        /// a.insert(1);
        /// a.insert(2);
        ///
        /// let mut b = BTreeSet::new();
        /// b.insert(2);
        /// b.insert(3);
        ///
        /// let diff: Vec<_> = a.difference(&b).cloned().collect();
        /// assert_eq!(diff, [1]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn difference<'a>(&'a self, other: &'a BTreeSet<T>) -> Difference<'a, T> {
            Difference {
                a: self.iter().peekable(),
                b: other.iter().peekable(),
            }
        }

        /// Visits the values representing the symmetric difference,
        /// i.e. the values that are in `self` or in `other` but not in both,
        /// in ascending order.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut a = BTreeSet::new();
        /// a.insert(1);
        /// a.insert(2);
        ///
        /// let mut b = BTreeSet::new();
        /// b.insert(2);
        /// b.insert(3);
        ///
        /// let sym_diff: Vec<_> = a.symmetric_difference(&b).cloned().collect();
        /// assert_eq!(sym_diff, [1, 3]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn symmetric_difference<'a>(&'a self,
                                        other: &'a BTreeSet<T>)
                                        -> SymmetricDifference<'a, T> {
            SymmetricDifference {
                a: self.iter().peekable(),
                b: other.iter().peekable(),
            }
        }

        /// Visits the values representing the intersection,
        /// i.e. the values that are both in `self` and `other`,
        /// in ascending order.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut a = BTreeSet::new();
        /// a.insert(1);
        /// a.insert(2);
        ///
        /// let mut b = BTreeSet::new();
        /// b.insert(2);
        /// b.insert(3);
        ///
        /// let intersection: Vec<_> = a.intersection(&b).cloned().collect();
        /// assert_eq!(intersection, [2]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn intersection<'a>(&'a self, other: &'a BTreeSet<T>) -> Intersection<'a, T> {
            Intersection {
                a: self.iter().peekable(),
                b: other.iter().peekable(),
            }
        }

        /// Visits the values representing the union,
        /// i.e. all the values in `self` or `other`, without duplicates,
        /// in ascending order.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut a = BTreeSet::new();
        /// a.insert(1);
        ///
        /// let mut b = BTreeSet::new();
        /// b.insert(2);
        ///
        /// let union: Vec<_> = a.union(&b).cloned().collect();
        /// assert_eq!(union, [1, 2]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn union<'a>(&'a self, other: &'a BTreeSet<T>) -> Union<'a, T> {
            Union {
                a: self.iter().peekable(),
                b: other.iter().peekable(),
            }
        }

        /// Clears the set, removing all values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut v = BTreeSet::new();
        /// v.insert(1);
        /// v.clear();
        /// assert!(v.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn clear(&mut self) {
            self.map.clear()
        }

        /// Returns `true` if the set contains a value.
        ///
        /// The value may be any borrowed form of the set's value type,
        /// but the ordering on the borrowed form *must* match the
        /// ordering on the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let set: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.contains(&1), true);
        /// assert_eq!(set.contains(&4), false);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
            where T: Borrow<Q>,
                  Q: Ord
        {
            self.map.contains_key(value)
        }

        /// Returns a reference to the value in the set, if any, that is equal to the given value.
        ///
        /// The value may be any borrowed form of the set's value type,
        /// but the ordering on the borrowed form *must* match the
        /// ordering on the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let set: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.get(&2), Some(&2));
        /// assert_eq!(set.get(&4), None);
        /// ```
        #[stable(feature = "set_recovery", since = "1.9.0")]
        pub fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T>
            where T: Borrow<Q>,
                  Q: Ord
        {
            Recover::get(&self.map, value)
        }

        /// Returns `true` if `self` has no elements in common with `other`.
        /// This is equivalent to checking for an empty intersection.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let a: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let mut b = BTreeSet::new();
        ///
        /// assert_eq!(a.is_disjoint(&b), true);
        /// b.insert(4);
        /// assert_eq!(a.is_disjoint(&b), true);
        /// b.insert(1);
        /// assert_eq!(a.is_disjoint(&b), false);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_disjoint(&self, other: &BTreeSet<T>) -> bool {
            self.intersection(other).next().is_none()
        }

        /// Returns `true` if the set is a subset of another,
        /// i.e. `other` contains at least all the values in `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let sup: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
        /// let mut set = BTreeSet::new();
        ///
        /// assert_eq!(set.is_subset(&sup), true);
        /// set.insert(2);
        /// assert_eq!(set.is_subset(&sup), true);
        /// set.insert(4);
        /// assert_eq!(set.is_subset(&sup), false);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_subset(&self, other: &BTreeSet<T>) -> bool {
            // Stolen from TreeMap
            let mut x = self.iter();
            let mut y = other.iter();
            let mut a = x.next();
            let mut b = y.next();
            while a.is_some() {
                if b.is_none() {
                    return false;
                }

                let a1 = a.unwrap();
                let b1 = b.unwrap();

                match b1.cmp(a1) {
                    Less => (),
                    Greater => return false,
                    Equal => a = x.next(),
                }

                b = y.next();
            }
            true
        }

        /// Returns `true` if the set is a superset of another,
        /// i.e. `self` contains at least all the values in `other`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let sub: BTreeSet<_> = [1, 2].iter().cloned().collect();
        /// let mut set = BTreeSet::new();
        ///
        /// assert_eq!(set.is_superset(&sub), false);
        ///
        /// set.insert(0);
        /// set.insert(1);
        /// assert_eq!(set.is_superset(&sub), false);
        ///
        /// set.insert(2);
        /// assert_eq!(set.is_superset(&sub), true);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_superset(&self, other: &BTreeSet<T>) -> bool {
            other.is_subset(self)
        }

        /// Adds a value to the set.
        ///
        /// If the set did not have this value present, `true` is returned.
        ///
        /// If the set did have this value present, `false` is returned, and the
        /// entry is not updated. See the [module-level documentation] for more.
        ///
        /// [module-level documentation]: index.html#insert-and-complex-keys
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut set = BTreeSet::new();
        ///
        /// assert_eq!(set.insert(2), true);
        /// assert_eq!(set.insert(2), false);
        /// assert_eq!(set.len(), 1);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(&mut self, value: T) -> bool {
            self.map.insert(value, ()).is_none()
        }

        /// Adds a value to the set, replacing the existing value, if any, that is equal to the given
        /// one. Returns the replaced value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut set = BTreeSet::new();
        /// set.insert(Vec::<i32>::new());
        ///
        /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 0);
        /// set.replace(Vec::with_capacity(10));
        /// assert_eq!(set.get(&[][..]).unwrap().capacity(), 10);
        /// ```
        #[stable(feature = "set_recovery", since = "1.9.0")]
        pub fn replace(&mut self, value: T) -> Option<T> {
            Recover::replace(&mut self.map, value)
        }

        /// Removes a value from the set. Returns `true` if the value was
        /// present in the set.
        ///
        /// The value may be any borrowed form of the set's value type,
        /// but the ordering on the borrowed form *must* match the
        /// ordering on the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut set = BTreeSet::new();
        ///
        /// set.insert(2);
        /// assert_eq!(set.remove(&2), true);
        /// assert_eq!(set.remove(&2), false);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool
            where T: Borrow<Q>,
                  Q: Ord
        {
            self.map.remove(value).is_some()
        }

        /// Removes and returns the value in the set, if any, that is equal to the given one.
        ///
        /// The value may be any borrowed form of the set's value type,
        /// but the ordering on the borrowed form *must* match the
        /// ordering on the value type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut set: BTreeSet<_> = [1, 2, 3].iter().cloned().collect();
        /// assert_eq!(set.take(&2), Some(2));
        /// assert_eq!(set.take(&2), None);
        /// ```
        #[stable(feature = "set_recovery", since = "1.9.0")]
        pub fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T>
            where T: Borrow<Q>,
                  Q: Ord
        {
            Recover::take(&mut self.map, value)
        }

        /// Moves all elements from `other` into `Self`, leaving `other` empty.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut a = BTreeSet::new();
        /// a.insert(1);
        /// a.insert(2);
        /// a.insert(3);
        ///
        /// let mut b = BTreeSet::new();
        /// b.insert(3);
        /// b.insert(4);
        /// b.insert(5);
        ///
        /// a.append(&mut b);
        ///
        /// assert_eq!(a.len(), 5);
        /// assert_eq!(b.len(), 0);
        ///
        /// assert!(a.contains(&1));
        /// assert!(a.contains(&2));
        /// assert!(a.contains(&3));
        /// assert!(a.contains(&4));
        /// assert!(a.contains(&5));
        /// ```
        #[stable(feature = "btree_append", since = "1.11.0")]
        pub fn append(&mut self, other: &mut Self) {
            self.map.append(&mut other.map);
        }

        /// Splits the collection into two at the given key. Returns everything after the given key,
        /// including the key.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut a = BTreeSet::new();
        /// a.insert(1);
        /// a.insert(2);
        /// a.insert(3);
        /// a.insert(17);
        /// a.insert(41);
        ///
        /// let b = a.split_off(&3);
        ///
        /// assert_eq!(a.len(), 2);
        /// assert_eq!(b.len(), 3);
        ///
        /// assert!(a.contains(&1));
        /// assert!(a.contains(&2));
        ///
        /// assert!(b.contains(&3));
        /// assert!(b.contains(&17));
        /// assert!(b.contains(&41));
        /// ```
        #[stable(feature = "btree_split_off", since = "1.11.0")]
        pub fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self where T: Borrow<Q> {
            BTreeSet { map: self.map.split_off(key) }
        }
    }

    impl<T> BTreeSet<T> {
        /// Gets an iterator that visits the values in the `BTreeSet` in ascending order.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let set: BTreeSet<usize> = [1, 2, 3].iter().cloned().collect();
        /// let mut set_iter = set.iter();
        /// assert_eq!(set_iter.next(), Some(&1));
        /// assert_eq!(set_iter.next(), Some(&2));
        /// assert_eq!(set_iter.next(), Some(&3));
        /// assert_eq!(set_iter.next(), None);
        /// ```
        ///
        /// Values returned by the iterator are returned in ascending order:
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let set: BTreeSet<usize> = [3, 1, 2].iter().cloned().collect();
        /// let mut set_iter = set.iter();
        /// assert_eq!(set_iter.next(), Some(&1));
        /// assert_eq!(set_iter.next(), Some(&2));
        /// assert_eq!(set_iter.next(), Some(&3));
        /// assert_eq!(set_iter.next(), None);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { iter: self.map.keys() }
        }

        /// Returns the number of elements in the set.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut v = BTreeSet::new();
        /// assert_eq!(v.len(), 0);
        /// v.insert(1);
        /// assert_eq!(v.len(), 1);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn len(&self) -> usize {
            self.map.len()
        }

        /// Returns `true` if the set contains no elements.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let mut v = BTreeSet::new();
        /// assert!(v.is_empty());
        /// v.insert(1);
        /// assert!(!v.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> FromIterator<T> for BTreeSet<T> {
        fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> BTreeSet<T> {
            let mut set = BTreeSet::new();
            set.extend(iter);
            set
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> IntoIterator for BTreeSet<T> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        /// Gets an iterator for moving out the `BTreeSet`'s contents.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let set: BTreeSet<usize> = [1, 2, 3, 4].iter().cloned().collect();
        ///
        /// let v: Vec<_> = set.into_iter().collect();
        /// assert_eq!(v, [1, 2, 3, 4]);
        /// ```
        fn into_iter(self) -> IntoIter<T> {
            IntoIter { iter: self.map.into_iter() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> IntoIterator for &'a BTreeSet<T> {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> Extend<T> for BTreeSet<T> {
        #[inline]
        fn extend<Iter: IntoIterator<Item = T>>(&mut self, iter: Iter) {
            for elem in iter {
                self.insert(elem);
            }
        }
    }

    #[stable(feature = "extend_ref", since = "1.2.0")]
    impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BTreeSet<T> {
        fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
            self.extend(iter.into_iter().cloned());
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> Default for BTreeSet<T> {
        /// Makes an empty `BTreeSet<T>` with a reasonable choice of B.
        fn default() -> BTreeSet<T> {
            BTreeSet::new()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T: Ord + Clone> Sub<&'b BTreeSet<T>> for &'a BTreeSet<T> {
        type Output = BTreeSet<T>;

        /// Returns the difference of `self` and `rhs` as a new `BTreeSet<T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: BTreeSet<_> = vec![3, 4, 5].into_iter().collect();
        ///
        /// let result = &a - &b;
        /// let result_vec: Vec<_> = result.into_iter().collect();
        /// assert_eq!(result_vec, [1, 2]);
        /// ```
        fn sub(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
            self.difference(rhs).cloned().collect()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T: Ord + Clone> BitXor<&'b BTreeSet<T>> for &'a BTreeSet<T> {
        type Output = BTreeSet<T>;

        /// Returns the symmetric difference of `self` and `rhs` as a new `BTreeSet<T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: BTreeSet<_> = vec![2, 3, 4].into_iter().collect();
        ///
        /// let result = &a ^ &b;
        /// let result_vec: Vec<_> = result.into_iter().collect();
        /// assert_eq!(result_vec, [1, 4]);
        /// ```
        fn bitxor(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
            self.symmetric_difference(rhs).cloned().collect()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T: Ord + Clone> BitAnd<&'b BTreeSet<T>> for &'a BTreeSet<T> {
        type Output = BTreeSet<T>;

        /// Returns the intersection of `self` and `rhs` as a new `BTreeSet<T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: BTreeSet<_> = vec![2, 3, 4].into_iter().collect();
        ///
        /// let result = &a & &b;
        /// let result_vec: Vec<_> = result.into_iter().collect();
        /// assert_eq!(result_vec, [2, 3]);
        /// ```
        fn bitand(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
            self.intersection(rhs).cloned().collect()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, 'b, T: Ord + Clone> BitOr<&'b BTreeSet<T>> for &'a BTreeSet<T> {
        type Output = BTreeSet<T>;

        /// Returns the union of `self` and `rhs` as a new `BTreeSet<T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::BTreeSet;
        ///
        /// let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
        /// let b: BTreeSet<_> = vec![3, 4, 5].into_iter().collect();
        ///
        /// let result = &a | &b;
        /// let result_vec: Vec<_> = result.into_iter().collect();
        /// assert_eq!(result_vec, [1, 2, 3, 4, 5]);
        /// ```
        fn bitor(self, rhs: &BTreeSet<T>) -> BTreeSet<T> {
            self.union(rhs).cloned().collect()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Debug> Debug for BTreeSet<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_set().entries(self.iter()).finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Iter<'a, T> {
        fn clone(&self) -> Iter<'a, T> {
            Iter { iter: self.iter.clone() }
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            self.iter.next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        fn next_back(&mut self) -> Option<&'a T> {
            self.iter.next_back()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> ExactSizeIterator for Iter<'a, T> {
        fn len(&self) -> usize { self.iter.len() }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T> FusedIterator for Iter<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Iterator for IntoIter<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            self.iter.next().map(|(k, _)| k)
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> DoubleEndedIterator for IntoIter<T> {
        fn next_back(&mut self) -> Option<T> {
            self.iter.next_back().map(|(k, _)| k)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ExactSizeIterator for IntoIter<T> {
        fn len(&self) -> usize { self.iter.len() }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for IntoIter<T> {}

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, T> Clone for Range<'a, T> {
        fn clone(&self) -> Range<'a, T> {
            Range { iter: self.iter.clone() }
        }
    }

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, T> Iterator for Range<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            self.iter.next().map(|(k, _)| k)
        }
    }

    #[stable(feature = "btree_range", since = "1.17.0")]
    impl<'a, T> DoubleEndedIterator for Range<'a, T> {
        fn next_back(&mut self) -> Option<&'a T> {
            self.iter.next_back().map(|(k, _)| k)
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T> FusedIterator for Range<'a, T> {}

    /// Compare `x` and `y`, but return `short` if x is None and `long` if y is None
    fn cmp_opt<T: Ord>(x: Option<&T>, y: Option<&T>, short: Ordering, long: Ordering) -> Ordering {
        match (x, y) {
            (None, _) => short,
            (_, None) => long,
            (Some(x1), Some(y1)) => x1.cmp(y1),
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Difference<'a, T> {
        fn clone(&self) -> Difference<'a, T> {
            Difference {
                a: self.a.clone(),
                b: self.b.clone(),
            }
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: Ord> Iterator for Difference<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            loop {
                match cmp_opt(self.a.peek(), self.b.peek(), Less, Less) {
                    Less => return self.a.next(),
                    Equal => {
                        self.a.next();
                        self.b.next();
                    }
                    Greater => {
                        self.b.next();
                    }
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let a_len = self.a.len();
            let b_len = self.b.len();
            (a_len.saturating_sub(b_len), Some(a_len))
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T: Ord> FusedIterator for Difference<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for SymmetricDifference<'a, T> {
        fn clone(&self) -> SymmetricDifference<'a, T> {
            SymmetricDifference {
                a: self.a.clone(),
                b: self.b.clone(),
            }
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: Ord> Iterator for SymmetricDifference<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            loop {
                match cmp_opt(self.a.peek(), self.b.peek(), Greater, Less) {
                    Less => return self.a.next(),
                    Equal => {
                        self.a.next();
                        self.b.next();
                    }
                    Greater => return self.b.next(),
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (0, Some(self.a.len() + self.b.len()))
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T: Ord> FusedIterator for SymmetricDifference<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Intersection<'a, T> {
        fn clone(&self) -> Intersection<'a, T> {
            Intersection {
                a: self.a.clone(),
                b: self.b.clone(),
            }
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: Ord> Iterator for Intersection<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            loop {
                match Ord::cmp(self.a.peek()?, self.b.peek()?) {
                    Less => {
                        self.a.next();
                    }
                    Equal => {
                        self.b.next();
                        return self.a.next();
                    }
                    Greater => {
                        self.b.next();
                    }
                }
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (0, Some(min(self.a.len(), self.b.len())))
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T: Ord> FusedIterator for Intersection<'a, T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Clone for Union<'a, T> {
        fn clone(&self) -> Union<'a, T> {
            Union {
                a: self.a.clone(),
                b: self.b.clone(),
            }
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: Ord> Iterator for Union<'a, T> {
        type Item = &'a T;

        fn next(&mut self) -> Option<&'a T> {
            match cmp_opt(self.a.peek(), self.b.peek(), Greater, Less) {
                Less => self.a.next(),
                Equal => {
                    self.b.next();
                    self.a.next()
                }
                Greater => self.b.next(),
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let a_len = self.a.len();
            let b_len = self.b.len();
            (max(a_len, b_len), Some(a_len + b_len))
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T: Ord> FusedIterator for Union<'a, T> {}

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