Function poll_multiple_step 

fn poll_multiple_step<I, P, S>(
    streams: I,
    cx: &mut Context<'_>,
    before: Option<&S::Ordering>,
    retry: Option<&mut Option<S::Ordering>>,
) -> Poll<PollResult<S::Ordering, S::Data>>
where
    I: IntoIterator<Item = Pin<P>>,
    P: DerefMut<Target = Peekable<S>>,
    S: OrderedStream,
    S::Ordering: Clone,