public function __construct(Client $client, array $attributes, array $properties)
 {
     parent::__construct($client, $attributes, $properties);
     /**
      * Register this subquestion with the parent.
      */
     if (!isset($this->parent)) {
         throw new \Exception("Parent is required.");
     }
     $this->parent->subQuestions[$this->dimension][] = $this;
 }