public function getRelativeComplementCollection(self $questionCollection)
 {
     $questionIds = array_flip($questionCollection->getInvolvedQuestionIds());
     $relativeComplementCollection = new self();
     foreach ($this->getQuestions() as $question) {
         if (!isset($questionIds[$question->getQuestionId()])) {
             $relativeComplementCollection->addQuestion($question);
         }
     }
     return $relativeComplementCollection;
 }