docs: def closeAndNotifyAction(self, request, queryset):
          ''' Close all submissions were the tutor sayed that the grading is finished,
              and inform the student. CLosing only graded submissions is a safeguard,
              since backend users tend to checkbox-mark all submissions without thinking.
          '''
          mails = []
          qs = queryset.filter(Q(state=Submission.GRADED))
          for subm in qs:
              subm.inform_student(request, Submission.CLOSED)
              mails.append(str(subm.pk))
          # works in bulk because inform_student never fails
          qs.update(state=Submission.CLOSED)
          if len(mails) == 0:
              self.message_user(request, "Nothing closed, no mails sent.")
          else:
              self.message_user(
                  request, "Mail sent for submissions: " + ",".join(mails))