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

/// A stream combinator which chains a computation onto values produced by a
/// stream.
///
/// This structure is produced by the `Stream::and_then` method.
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct AndThen<S, F, U>
where U: IntoFuture,
{
stream: S,
future: Option<U::Future>,
f: F,
}

pub fn new<S, F, U>(s: S, f: F) -> AndThen<S, F, U>
where S: Stream,
F: FnMut(S::Item) -> U,
U: IntoFuture<Error=S::Error>,
{
AndThen {
stream: s,
future: None,
f: f,
}
}

impl<S, F, U> AndThen<S, F, U>
where U: IntoFuture,
{
/// Acquires a reference to the underlying stream that this combinator is
/// pulling from.
pub fn get_ref(&self) -> &S {
&self.stream
}

/// Acquires a mutable reference to the underlying stream that this
/// combinator is pulling from.
///
/// Note that care must be taken to avoid tampering with the state of the
/// stream which may otherwise confuse this combinator.
pub fn get_mut(&mut self) -> &mut S {
&mut self.stream
}

/// Consumes this combinator, returning the underlying stream.
///
/// Note that this may discard intermediate state of this combinator, so
/// care should be taken to avoid losing resources when this is called.
pub fn into_inner(self) -> S {
self.stream
}
}

// Forwarding impl of Sink from the underlying stream
impl<S, F, U: IntoFuture> ::sink::Sink for AndThen<S, F, U>
where S: ::sink::Sink
{
type SinkItem = S::SinkItem;
type SinkError = S::SinkError;

fn start_send(&mut self, item: S::SinkItem) -> ::StartSend<S::SinkItem, S::SinkError> {
self.stream.start_send(item)
}

fn poll_complete(&mut self) -> Poll<(), S::SinkError> {
self.stream.poll_complete()
}

fn close(&mut self) -> Poll<(), S::SinkError> {
self.stream.close()
}
}

impl<S, F, U> Stream for AndThen<S, F, U>
where S: Stream,
F: FnMut(S::Item) -> U,
U: IntoFuture<Error=S::Error>,
{
type Item = U::Item;
type Error = S::Error;

fn poll(&mut self) -> Poll<Option<U::Item>, S::Error> {
if self.future.is_none() {
let item = match try_ready!(self.stream.poll()) {
Some(e) => e,
};
self.future = Some((self.f)(item).into_future());
}
assert!(self.future.is_some());
match self.future.as_mut().unwrap().poll() {