@abc.abstractmethod<EOL><INDENT>def get_grade_system(self):<DEDENT>
return<EOL>
Gets the ``GradeSystem`` in which this grade belongs.

        :return: the package grade system
        :rtype: ``osid.grading.GradeSystem``
        :raise: ``OperationFailed`` -- unable to complete request

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