Iterable of rule names used by :meth:`create`

        Args:
            attr (None or str): Name of the class attribute to which to get the
                names. If None, one of ``'_rules'``, ``'_binary_rules'`` is
                automatically chosen

def rules(cls, attr=None):
        """Iterable of rule names used by :meth:`create`

        Args:
            attr (None or str): Name of the class attribute to which to get the
                names. If None, one of ``'_rules'``, ``'_binary_rules'`` is
                automatically chosen
        """
        try:
            if attr is None:
                attr = cls._rules_attr()
            return getattr(cls, attr).keys()
        except TypeError:
            return ()