@abc.abstractmethod<EOL><INDENT>def get_grade_id(self):<DEDENT>
return<EOL>
Gets the grade ``Id`` in this entry if the grading system is based on grades.

        :return: the grade ``Id``
        :rtype: ``osid.id.Id``
        :raise: ``IllegalState`` -- ``is_graded()`` is ``false`` or ``GradeSystem.isBasedOnGrades()`` is ``false``

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