docs: def unassign_grade_system_from_gradebook(self, grade_system_id, gradebook_id):
          """Removes a ``GradeSystem`` from a ``Gradebook``.
  
          arg:    grade_system_id (osid.id.Id): the ``Id`` of the
                  ``GradeSystem``
          arg:    gradebook_id (osid.id.Id): the ``Id`` of the
                  ``Gradebook``
          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.*
  
          """
          # Implemented from template for
          # osid.resource.ResourceBinAssignmentSession.unassign_resource_from_bin
          mgr = self._get_provider_manager('GRADING', local=True)
          lookup_session = mgr.get_gradebook_lookup_session(proxy=self._proxy)
          lookup_session.get_gradebook(gradebook_id)  # to raise NotFound
          self._unassign_object_from_catalog(grade_system_id, gradebook_id)