def _get_consecutive_portions_of_front(front):
    """
    Yields lists of the form [(f, s), (f, s)], one at a time from the given front (which is a list of the same form),
    such that each list yielded is consecutive in frequency.
    """
    last_f = None
    ls = []
    for f, s in front:
        if last_f is not None and f != last_f + 1:
            yield ls
            ls = []
        ls.append((f, s))
        last_f = f
    yield ls
