/**
  * Cancel a booking
  * @param type $a_usr_id
  * @param type $a_app_id
  * @return bool
  */
 public static function cancelBooking($a_usr_id, $a_app_id, $a_send_notification = true)
 {
     // Delete personal copy of appointment
     include_once './Services/Calendar/classes/class.ilCalendarEntry.php';
     $app = new ilCalendarEntry($a_app_id);
     include_once './Services/Calendar/classes/ConsultationHours/class.ilConsultationHourAppointments.php';
     $user_apps = ilConsultationHourAppointments::getAppointmentIds($a_usr_id, $app->getContextId(), $app->getStart(), ilCalendarCategory::TYPE_CH, false);
     foreach ($user_apps as $uapp_id) {
         $uapp = new ilCalendarEntry($uapp_id);
         $uapp->delete();
         include_once './Services/Calendar/classes/class.ilCalendarCategoryAssignments.php';
         ilCalendarCategoryAssignments::_deleteByAppointmentId($uapp_id);
         break;
     }
     // Delete booking entries
     // Send notification
     $booking = new ilBookingEntry($app->getContextId());
     if ($a_send_notification) {
         $booking->cancelBooking($a_app_id, $a_usr_id);
     } else {
         $booking->deleteBooking($a_app_id, $a_usr_id);
     }
     return true;
 }