'List users 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 users or an empty list. See user schema in
:class:`~.IdentityDriverBase`.
:rtype: list of dict'

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

    raise exception.NotImplemented()
