public function __construct(Timetable $timetable)
 {
     parent::__construct($timetable);
     if (is_null($this->timetable)) {
         $this->throwException('No timetable loaded');
     }
 }
 public function __construct(Timetable $timetable)
 {
     if (is_null($timetable)) {
         $this->throwException("No timetable set");
     }
     $this->timetable = $timetable;
     parent::__construct();
 }