def _yield_finished_futures(fs, waiter, ref_collect):
    """
    Iterate on the list *fs*, yielding finished futures one by one in
    reverse order.
    Before yielding a future, *waiter* is removed from its waiters
    and the future is removed from each set in the collection of sets
    *ref_collect*.

    The aim of this function is to avoid keeping stale references after
    the future is yielded and before the iterator resumes.
    """
    while fs:
        f = fs[-1]
        for futures_set in ref_collect:
            futures_set.remove(f)
        with f._condition:
            f._waiters.remove(waiter)
        del f
        yield fs.pop()
