def pop_smallest():
    """Return and remove the (item, score) pair with lowest score.

        If len(self) is 0, raise IndexError.

        To be cleaer, this is the lowest score among the N best-scoring
        seen so far.  This is most useful if the capacity of the NBest
        object is never exceeded, in which case  pop_smallest() allows
        using the object as an ordinary smallest-in-first-out priority
        queue.
        """
