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    use {Future, Poll, Async};

    /// Future for the `select` combinator, waiting for one of two futures to
    /// complete.
    ///
    /// This is created by the `Future::select` method.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct Select<A, B> where A: Future, B: Future<Item=A::Item, Error=A::Error> {
        inner: Option<(A, B)>,
    }

    /// Future yielded as the second result in a `Select` future.
    ///
    /// This sentinel future represents the completion of the second future to a
    /// `select` which finished second.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct SelectNext<A, B> where A: Future, B: Future<Item=A::Item, Error=A::Error> {
        inner: OneOf<A, B>,
    }

    #[derive(Debug)]
    enum OneOf<A, B> where A: Future, B: Future {
        A(A),
        B(B),
    }

    pub fn new<A, B>(a: A, b: B) -> Select<A, B>
        where A: Future,
              B: Future<Item=A::Item, Error=A::Error>
    {
        Select {
            inner: Some((a, b)),
        }
    }

    impl<A, B> Future for Select<A, B>
        where A: Future,
              B: Future<Item=A::Item, Error=A::Error>,
    {
        type Item = (A::Item, SelectNext<A, B>);
        type Error = (A::Error, SelectNext<A, B>);

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            let (ret, is_a) = match self.inner {
                Some((ref mut a, ref mut b)) => {
                    match a.poll() {
                        Err(a) => (Err(a), true),
                        Ok(Async::Ready(a)) => (Ok(a), true),
                        Ok(Async::NotReady) => {
                            match b.poll() {
                                Err(a) => (Err(a), false),
                                Ok(Async::Ready(a)) => (Ok(a), false),
                                Ok(Async::NotReady) => return Ok(Async::NotReady),
                            }
                        }
                    }
                }
                None => panic!("cannot poll select twice"),
            };

            let (a, b) = self.inner.take().unwrap();
            let next = if is_a {OneOf::B(b)} else {OneOf::A(a)};
            let next = SelectNext { inner: next };
            match ret {
                Ok(a) => Ok(Async::Ready((a, next))),
                Err(e) => Err((e, next)),
            }
        }
    }

    impl<A, B> Future for SelectNext<A, B>
        where A: Future,
              B: Future<Item=A::Item, Error=A::Error>,
    {
        type Item = A::Item;
        type Error = A::Error;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            match self.inner {
                OneOf::A(ref mut a) => a.poll(),
                OneOf::B(ref mut b) => b.poll(),
            }
        }
    }

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