@abc.abstractmethod<EOL><INDENT>def get_relation_types(self):<DEDENT>
return<EOL>
Gets all known relation ``Types``.

        A relation Types relates two ``Types``.

        :return: known relation types
        :rtype: ``osid.type.TypeList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*