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    use core::mem;

    use {Future, Poll, IntoFuture, Async};
    use stream::Stream;

    /// A future used to collect all the results of a stream into one generic type.
    ///
    /// This future is returned by the `Stream::fold` method.
    #[derive(Debug)]
    #[must_use = "streams do nothing unless polled"]
    pub struct Fold<S, F, Fut, T> where Fut: IntoFuture {
        stream: S,
        f: F,
        state: State<T, Fut::Future>,
    }

    #[derive(Debug)]
    enum State<T, F> where F: Future {
        /// Placeholder state when doing work
        Empty,

        /// Ready to process the next stream item; current accumulator is the `T`
        Ready(T),

        /// Working on a future the process the previous stream item
        Processing(F),
    }

    pub fn new<S, F, Fut, T>(s: S, f: F, t: T) -> Fold<S, F, Fut, T>
        where S: Stream,
              F: FnMut(T, S::Item) -> Fut,
              Fut: IntoFuture<Item = T>,
              S::Error: From<Fut::Error>,
    {
        Fold {
            stream: s,
            f: f,
            state: State::Ready(t),
        }
    }

    impl<S, F, Fut, T> Future for Fold<S, F, Fut, T>
        where S: Stream,
              F: FnMut(T, S::Item) -> Fut,
              Fut: IntoFuture<Item = T>,
              S::Error: From<Fut::Error>,
    {
        type Item = T;
        type Error = S::Error;

        fn poll(&mut self) -> Poll<T, S::Error> {
            loop {
                match mem::replace(&mut self.state, State::Empty) {
                    State::Empty => panic!("cannot poll Fold twice"),
                    State::Ready(state) => {
                        match self.stream.poll()? {
                            Async::Ready(Some(e)) => {
                                let future = (self.f)(state, e);
                                let future = future.into_future();
                                self.state = State::Processing(future);
                            }
                            Async::Ready(None) => return Ok(Async::Ready(state)),
                            Async::NotReady => {
                                self.state = State::Ready(state);
                                return Ok(Async::NotReady)
                            }
                        }
                    }
                    State::Processing(mut fut) => {
                        match fut.poll()? {
                            Async::Ready(state) => self.state = State::Ready(state),
                            Async::NotReady => {
                                self.state = State::Processing(fut);
                                return Ok(Async::NotReady)
                            }
                        }
                    }
                }
            }
        }
    }

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