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

        :return: the list of grades ``Ids``
        :rtype: ``osid.id.IdList``
        :raise: ``IllegalState`` -- ``is_based_on_grades()`` is ``false``

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