function participantTypeString()
 {
     if ($this->ParticipantType < self::TYPE_CUSTOM) {
         return eZCollaborationItemParticipantLink::typeString($this->ParticipantType);
     }
     $item = eZCollaborationItem::fetch($this->CollaborationID);
     return $item->attribute('type_identifier') . '_' . $item->handler()->participantTypeString($this->ParticipantType);
 }