def get_relation_types(self):
raise Unimplemented()<EOL>
Gets all known relation ``Types``.

        A relation Types relates two ``Types``.

        return: (osid.type.TypeList) - known relation types
        raise:  OperationFailed - unable to complete request
        raise:  PermissionDenied - authorization failure
        *compliance: mandatory -- This method must be implemented.*