def _has_role(self, identifiers, role_s):
        """
        :type identifiers:  subject_abcs.IdentifierCollection
        :type role_s: Set of String(s)
        """
        for realm in self.realms:
            # the realm's has_role returns a generator
            yield from realm.has_role(identifiers, role_s)
:type identifiers:  subject_abcs.IdentifierCollection
        :type role_s: Set of String(s)