@classmethod
def parse(cls, rules):
    """Create a `PluralRule` instance for the given rules.  If the rules
        are a `PluralRule` object, that object is returned.

        :param rules: the rules as list or dict, or a `PluralRule` object
        :raise RuleError: if the expression is malformed
        """
    if isinstance(rules, cls):
        return rules
    return cls(rules)
