/**
  * check if the user is allowed to actually read the calendar
  * @param ICalendar $calendar
  * @throws BusinessLayer\Exception
  */
 private function checkAllowedToRead(ICalendar $calendar)
 {
     if (!$calendar->doesAllow(Permissions::READ)) {
         throw new BusinessLayer\Exception('Not allowed to read calendar', HTTP::STATUS_FORBIDDEN);
     }
 }