@classmethod
def _get_precedence(cls, acceptance):
    """Return numeric precedence for given acceptance object."""
    try:
        number = cls._precedence[frozenset(acceptance.scope)]
    except KeyError:
        import warnings
        message = 'unrecognized scope {0!r}'.format(acceptance.scope)
        warnings.warn(message.format(message), UserWarning)
        number = 0
    return number
