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    use core::mem;

    use {Future, Poll, Async};

    #[derive(Debug)]
    pub enum Chain<A, B, C> where A: Future {
        First(A, C),
        Second(B),
        Done,
    }

    impl<A, B, C> Chain<A, B, C>
        where A: Future,
              B: Future,
    {
        pub fn new(a: A, c: C) -> Chain<A, B, C> {
            Chain::First(a, c)
        }

        pub fn poll<F>(&mut self, f: F) -> Poll<B::Item, B::Error>
            where F: FnOnce(Result<A::Item, A::Error>, C)
                            -> Result<Result<B::Item, B>, B::Error>,
        {
            let a_result = match *self {
                Chain::First(ref mut a, _) => {
                    match a.poll() {
                        Ok(Async::NotReady) => return Ok(Async::NotReady),
                        Ok(Async::Ready(t)) => Ok(t),
                        Err(e) => Err(e),
                    }
                }
                Chain::Second(ref mut b) => return b.poll(),
                Chain::Done => panic!("cannot poll a chained future twice"),
            };
            let data = match mem::replace(self, Chain::Done) {
                Chain::First(_, c) => c,
                _ => panic!(),
            };
            match f(a_result, data)? {
                Ok(e) => Ok(Async::Ready(e)),
                Err(mut b) => {
                    let ret = b.poll();
                    *self = Chain::Second(b);
                    ret
                }
            }
        }
    }

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