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    use core::marker::PhantomData;

    use {Future, Poll, Async};

    /// Future for the `from_err` combinator, changing the error type of a future.
    ///
    /// This is created by the `Future::from_err` method.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct FromErr<A, E> where A: Future {
        future: A,
        f: PhantomData<E>
    }

    pub fn new<A, E>(future: A) -> FromErr<A, E>
        where A: Future
    {
        FromErr {
            future: future,
            f: PhantomData
        }
    }

    impl<A:Future, E:From<A::Error>> Future for FromErr<A, E> {
        type Item = A::Item;
        type Error = E;

        fn poll(&mut self) -> Poll<A::Item, E> {
            let e = match self.future.poll() {
                Ok(Async::NotReady) => return Ok(Async::NotReady),
                other => other,
            };
            e.map_err(From::from)
        }
    }

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