def _decrement_lower_bound(interval):
    """
    Decrement the lower bound of a possibly inverted interval.
    i.e. turn (1, 5) into (0, 5) and (5, 1) into (5, 0).
    Intervals in m8 alignment files can be inverted.
    """
    (bound_one, bound_two) = interval
    if bound_one < bound_two:
        return (bound_one - 1, bound_two)
    else:
        return (bound_one, bound_two - 1)