@abc.abstractmethod<EOL><INDENT>def unassign_grade_system_from_gradebook(self, grade_system_id, gradebook_id):<DEDENT>
pass<EOL>
Removes a ``GradeSystem`` from a ``Gradebook``.

        :param grade_system_id: the ``Id`` of the ``GradeSystem``
        :type grade_system_id: ``osid.id.Id``
        :param gradebook_id: the ``Id`` of the ``Gradebook``
        :type gradebook_id: ``osid.id.Id``
        :raise: ``NotFound`` -- ``grade_system_id`` or ``gradebook_id`` not found or ``grade_system_id`` not assigned to ``gradebook_id``
        :raise: ``NullArgument`` -- ``grade_system_id`` or ``gradebook_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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