'List groups in the system.
:param hints: filter hints which the driver should
implement if at all possible.
:type hints: keystone.common.driver_hints.Hints
:returns: a list of group_refs or an empty list. See group schema in
:class:`~.IdentityDriverBase`.'

@abc.abstractmethod
def list_groups(self, hints):

    raise exception.NotImplemented()
