def is_metric_cls(cls):
    """A metric class is defined as follows:

        - It inherits DiffParserBase
        - It is not DiffParserBase
        - It does not have __metric__ = False
    """
    return (
        cls is not DiffParserBase and
        cls.__dict__.get('__metric__', True) and
        issubclass(cls, DiffParserBase)
    )