public function add(QuestionSheet $sheet)
 {
     if (NULLUser::getInstance() === $sheet->getOwner()) {
         throw new \InvalidArgumentException("incorrect user");
     }
     $this->entityManager->persist($sheet);
     $this->entityManager->flush();
 }
 protected function check_delete(QuestionSheet $sheet, User $user)
 {
     return $user->isTheSameAs($sheet->getOwner());
 }