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    use core::mem;
    use core::fmt::{Debug, Formatter, Result as FmtResult};
    use core::default::Default;

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

    /// A stream combinator to concatenate the results of a stream into the first
    /// yielded item.
    ///
    /// This structure is produced by the `Stream::concat` method.
    #[must_use = "streams do nothing unless polled"]
    pub struct Concat2<S>
        where S: Stream,
    {
        inner: ConcatSafe<S>
    }

    impl<S: Debug> Debug for Concat2<S> where S: Stream, S::Item: Debug {
        fn fmt(&self, fmt: &mut Formatter) -> FmtResult {
            fmt.debug_struct("Concat2")
                .field("inner", &self.inner)
                .finish()
        }
    }

    pub fn new2<S>(s: S) -> Concat2<S>
        where S: Stream,
              S::Item: Extend<<<S as Stream>::Item as IntoIterator>::Item> + IntoIterator + Default,
    {
        Concat2 {
            inner: new_safe(s)
        }
    }

    impl<S> Future for Concat2<S>
        where S: Stream,
              S::Item: Extend<<<S as Stream>::Item as IntoIterator>::Item> + IntoIterator + Default,

    {
        type Item = S::Item;
        type Error = S::Error;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            self.inner.poll().map(|a| {
                match a {
                    Async::NotReady => Async::NotReady,
                    Async::Ready(None) => Async::Ready(Default::default()),
                    Async::Ready(Some(e)) => Async::Ready(e)
                }
            })
        }
    }

    /// A stream combinator to concatenate the results of a stream into the first
    /// yielded item.
    ///
    /// This structure is produced by the `Stream::concat` method.
    #[deprecated(since="0.1.18", note="please use `Stream::Concat2` instead")]
    #[must_use = "streams do nothing unless polled"]
    pub struct Concat<S>
        where S: Stream,
    {
        inner: ConcatSafe<S>
    }

    #[allow(deprecated)]
    impl<S: Debug> Debug for Concat<S> where S: Stream, S::Item: Debug {
        fn fmt(&self, fmt: &mut Formatter) -> FmtResult {
            fmt.debug_struct("Concat")
                .field("inner", &self.inner)
                .finish()
        }
    }

    #[allow(deprecated)]
    pub fn new<S>(s: S) -> Concat<S>
        where S: Stream,
              S::Item: Extend<<<S as Stream>::Item as IntoIterator>::Item> + IntoIterator,
    {
        Concat {
            inner: new_safe(s)
        }
    }

    #[allow(deprecated)]
    impl<S> Future for Concat<S>
        where S: Stream,
              S::Item: Extend<<<S as Stream>::Item as IntoIterator>::Item> + IntoIterator,

    {
        type Item = S::Item;
        type Error = S::Error;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            self.inner.poll().map(|a| {
                match a {
                    Async::NotReady => Async::NotReady,
                    Async::Ready(None) => panic!("attempted concatenation of empty stream"),
                    Async::Ready(Some(e)) => Async::Ready(e)
                }
            })
        }
    }

    #[derive(Debug)]
    struct ConcatSafe<S>
        where S: Stream,
    {
        stream: S,
        extend: Inner<S::Item>,
    }

    fn new_safe<S>(s: S) -> ConcatSafe<S>
        where S: Stream,
              S::Item: Extend<<<S as Stream>::Item as IntoIterator>::Item> + IntoIterator,
    {
        ConcatSafe {
            stream: s,
            extend: Inner::First,
        }
    }

    impl<S> Future for ConcatSafe<S>
        where S: Stream,
              S::Item: Extend<<<S as Stream>::Item as IntoIterator>::Item> + IntoIterator,

    {
        type Item = Option<S::Item>;
        type Error = S::Error;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            loop {
                match self.stream.poll() {
                    Ok(Async::Ready(Some(i))) => {
                        match self.extend {
                            Inner::First => {
                                self.extend = Inner::Extending(i);
                            },
                            Inner::Extending(ref mut e) => {
                                e.extend(i);
                            },
                            Inner::Done => unreachable!(),
                        }
                    },
                    Ok(Async::Ready(None)) => {
                        match mem::replace(&mut self.extend, Inner::Done) {
                            Inner::First => return Ok(Async::Ready(None)),
                            Inner::Extending(e) => return Ok(Async::Ready(Some(e))),
                            Inner::Done => panic!("cannot poll Concat again")
                        }
                    },
                    Ok(Async::NotReady) => return Ok(Async::NotReady),
                    Err(e) => {
                        self.extend = Inner::Done;
                        return Err(e)
                    }
                }
            }
        }
    }

    #[derive(Debug)]
    enum Inner<E> {
        First,
        Extending(E),
        Done,
    }

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