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    use crate::ops::Try;
    use crate::iter::LoopState;

    /// An iterator able to yield elements from both ends.
    ///
    /// Something that implements `DoubleEndedIterator` has one extra capability
    /// over something that implements [`Iterator`]: the ability to also take
    /// `Item`s from the back, as well as the front.
    ///
    /// It is important to note that both back and forth work on the same range,
    /// and do not cross: iteration is over when they meet in the middle.
    ///
    /// In a similar fashion to the [`Iterator`] protocol, once a
    /// `DoubleEndedIterator` returns `None` from a `next_back()`, calling it again
    /// may or may not ever return `Some` again. `next()` and `next_back()` are
    /// interchangeable for this purpose.
    ///
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let numbers = vec![1, 2, 3, 4, 5, 6];
    ///
    /// let mut iter = numbers.iter();
    ///
    /// assert_eq!(Some(&1), iter.next());
    /// assert_eq!(Some(&6), iter.next_back());
    /// assert_eq!(Some(&5), iter.next_back());
    /// assert_eq!(Some(&2), iter.next());
    /// assert_eq!(Some(&3), iter.next());
    /// assert_eq!(Some(&4), iter.next());
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next_back());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait DoubleEndedIterator: Iterator {
        /// Removes and returns an element from the end of the iterator.
        ///
        /// Returns `None` when there are no more elements.
        ///
        /// The [trait-level] docs contain more details.
        ///
        /// [trait-level]: trait.DoubleEndedIterator.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let numbers = vec![1, 2, 3, 4, 5, 6];
        ///
        /// let mut iter = numbers.iter();
        ///
        /// assert_eq!(Some(&1), iter.next());
        /// assert_eq!(Some(&6), iter.next_back());
        /// assert_eq!(Some(&5), iter.next_back());
        /// assert_eq!(Some(&2), iter.next());
        /// assert_eq!(Some(&3), iter.next());
        /// assert_eq!(Some(&4), iter.next());
        /// assert_eq!(None, iter.next());
        /// assert_eq!(None, iter.next_back());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn next_back(&mut self) -> Option<Self::Item>;

        /// Returns the `n`th element from the end of the iterator.
        ///
        /// This is essentially the reversed version of [`nth`]. Although like most indexing
        /// operations, the count starts from zero, so `nth_back(0)` returns the first value fro
        /// the end, `nth_back(1)` the second, and so on.
        ///
        /// Note that all elements between the end and the returned element will be
        /// consumed, including the returned element. This also means that calling
        /// `nth_back(0)` multiple times on the same iterator will return different
        /// elements.
        ///
        /// `nth_back()` will return [`None`] if `n` is greater than or equal to the length of the
        /// iterator.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        /// [`nth`]: ../../std/iter/trait.Iterator.html#method.nth
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(iter_nth_back)]
        /// let a = [1, 2, 3];
        /// assert_eq!(a.iter().nth_back(2), Some(&1));
        /// ```
        ///
        /// Calling `nth_back()` multiple times doesn't rewind the iterator:
        ///
        /// ```
        /// #![feature(iter_nth_back)]
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert_eq!(iter.nth_back(1), Some(&2));
        /// assert_eq!(iter.nth_back(1), None);
        /// ```
        ///
        /// Returning `None` if there are less than `n + 1` elements:
        ///
        /// ```
        /// #![feature(iter_nth_back)]
        /// let a = [1, 2, 3];
        /// assert_eq!(a.iter().nth_back(10), None);
        /// ```
        #[inline]
        #[unstable(feature = "iter_nth_back", issue = "56995")]
        fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
            for x in self.rev() {
                if n == 0 { return Some(x) }
                n -= 1;
            }
            None
        }

        /// This is the reverse version of [`try_fold()`]: it takes elements
        /// starting from the back of the iterator.
        ///
        /// [`try_fold()`]: trait.Iterator.html#method.try_fold
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = ["1", "2", "3"];
        /// let sum = a.iter()
        ///     .map(|&s| s.parse::<i32>())
        ///     .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
        /// assert_eq!(sum, Ok(6));
        /// ```
        ///
        /// Short-circuiting:
        ///
        /// ```
        /// let a = ["1", "rust", "3"];
        /// let mut it = a.iter();
        /// let sum = it
        ///     .by_ref()
        ///     .map(|&s| s.parse::<i32>())
        ///     .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
        /// assert!(sum.is_err());
        ///
        /// // Because it short-circuited, the remaining elements are still
        /// // available through the iterator.
        /// assert_eq!(it.next_back(), Some(&"1"));
        /// ```
        #[inline]
        #[stable(feature = "iterator_try_fold", since = "1.27.0")]
        fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok=B>
        {
            let mut accum = init;
            while let Some(x) = self.next_back() {
                accum = f(accum, x)?;
            }
            Try::from_ok(accum)
        }

        /// An iterator method that reduces the iterator's elements to a single,
        /// final value, starting from the back.
        ///
        /// This is the reverse version of [`fold()`]: it takes elements starting from
        /// the back of the iterator.
        ///
        /// `rfold()` takes two arguments: an initial value, and a closure with two
        /// arguments: an 'accumulator', and an element. The closure returns the value that
        /// the accumulator should have for the next iteration.
        ///
        /// The initial value is the value the accumulator will have on the first
        /// call.
        ///
        /// After applying this closure to every element of the iterator, `rfold()`
        /// returns the accumulator.
        ///
        /// This operation is sometimes called 'reduce' or 'inject'.
        ///
        /// Folding is useful whenever you have a collection of something, and want
        /// to produce a single value from it.
        ///
        /// [`fold()`]: trait.Iterator.html#method.fold
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// // the sum of all of the elements of a
        /// let sum = a.iter()
        ///            .rfold(0, |acc, &x| acc + x);
        ///
        /// assert_eq!(sum, 6);
        /// ```
        ///
        /// This example builds a string, starting with an initial value
        /// and continuing with each element from the back until the front:
        ///
        /// ```
        /// let numbers = [1, 2, 3, 4, 5];
        ///
        /// let zero = "0".to_string();
        ///
        /// let result = numbers.iter().rfold(zero, |acc, &x| {
        ///     format!("({} + {})", x, acc)
        /// });
        ///
        /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
        /// ```
        #[inline]
        #[stable(feature = "iter_rfold", since = "1.27.0")]
        fn rfold<B, F>(mut self, accum: B, mut f: F) -> B
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> B,
        {
            self.try_rfold(accum, move |acc, x| Ok::<B, !>(f(acc, x))).unwrap()
        }

        /// Searches for an element of an iterator from the back that satisfies a predicate.
        ///
        /// `rfind()` takes a closure that returns `true` or `false`. It applies
        /// this closure to each element of the iterator, starting at the end, and if any
        /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
        /// `false`, it returns [`None`].
        ///
        /// `rfind()` is short-circuiting; in other words, it will stop processing
        /// as soon as the closure returns `true`.
        ///
        /// Because `rfind()` takes a reference, and many iterators iterate over
        /// references, this leads to a possibly confusing situation where the
        /// argument is a double reference. You can see this effect in the
        /// examples below, with `&&x`.
        ///
        /// [`Some(element)`]: ../../std/option/enum.Option.html#variant.Some
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
        ///
        /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
        /// ```
        ///
        /// Stopping at the first `true`:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
        ///
        /// // we can still use `iter`, as there are more elements.
        /// assert_eq!(iter.next_back(), Some(&1));
        /// ```
        #[inline]
        #[stable(feature = "iter_rfind", since = "1.27.0")]
        fn rfind<P>(&mut self, mut predicate: P) -> Option<Self::Item>
        where
            Self: Sized,
            P: FnMut(&Self::Item) -> bool
        {
            self.try_rfold((), move |(), x| {
                if predicate(&x) { LoopState::Break(x) }
                else { LoopState::Continue(()) }
            }).break_value()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
        fn next_back(&mut self) -> Option<I::Item> {
            (**self).next_back()
        }
        fn nth_back(&mut self, n: usize) -> Option<I::Item> {
            (**self).nth_back(n)
        }
    }

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