@abc.abstractmethod<EOL><INDENT>def get_grade_systems(self):<DEDENT>
return<EOL>
Gets all ``GradeSystems``.

        In plenary mode, the returned list contains all known grade
        systems or an error results. Otherwise, the returned list may
        contain only those grade systems that are accessible through
        this session.

        :return: a ``GradeSystemList``
        :rtype: ``osid.grading.GradeSystemList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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