@abc.abstractmethod<EOL><INDENT>def get_score_system_id(self):<DEDENT>
return<EOL>
Gets a score system ``Id`` for the assessment.

        :return: the grade system
        :rtype: ``osid.id.Id``
        :raise: ``IllegalState`` -- ``is_score()`` is ``false``

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