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    use {Poll, Async};
    use stream::{Stream, Fuse};

    /// An adapter for merging the output of two streams.
    ///
    /// The merged stream produces items from either of the underlying streams as
    /// they become available, and the streams are polled in a round-robin fashion.
    /// Errors, however, are not merged: you get at most one error at a time.
    #[derive(Debug)]
    #[must_use = "streams do nothing unless polled"]
    pub struct Select<S1, S2> {
        stream1: Fuse<S1>,
        stream2: Fuse<S2>,
        flag: bool,
    }

    pub fn new<S1, S2>(stream1: S1, stream2: S2) -> Select<S1, S2>
        where S1: Stream,
              S2: Stream<Item = S1::Item, Error = S1::Error>
    {
        Select {
            stream1: stream1.fuse(),
            stream2: stream2.fuse(),
            flag: false,
        }
    }

    impl<S1, S2> Stream for Select<S1, S2>
        where S1: Stream,
              S2: Stream<Item = S1::Item, Error = S1::Error>
    {
        type Item = S1::Item;
        type Error = S1::Error;

        fn poll(&mut self) -> Poll<Option<S1::Item>, S1::Error> {
            let (a, b) = if self.flag {
                (&mut self.stream2 as &mut Stream<Item=_, Error=_>,
                 &mut self.stream1 as &mut Stream<Item=_, Error=_>)
            } else {
                (&mut self.stream1 as &mut Stream<Item=_, Error=_>,
                 &mut self.stream2 as &mut Stream<Item=_, Error=_>)
            };
            self.flag = !self.flag;

            let a_done = match a.poll()? {
                Async::Ready(Some(item)) => return Ok(Some(item).into()),
                Async::Ready(None) => true,
                Async::NotReady => false,
            };

            match b.poll()? {
                Async::Ready(Some(item)) => {
                    // If the other stream isn't finished yet, give them a chance to
                    // go first next time as we pulled something off `b`.
                    if !a_done {
                        self.flag = !self.flag;
                    }
                    Ok(Some(item).into())
                }
                Async::Ready(None) if a_done => Ok(None.into()),
                Async::Ready(None) | Async::NotReady => Ok(Async::NotReady),
            }
        }
    }

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