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))
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.