@abc.abstractmethod<EOL><INDENT>def get_grades(self):<DEDENT>
return<EOL>
Gets the grades in this system ranked from highest to lowest.

        :return: the list of grades
        :rtype: ``osid.grading.GradeList``
        :raise: ``IllegalState`` -- ``is_based_on_grades()`` is ``false``
        :raise: ``OperationFailed`` -- unable to complete request

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