/**
  * add a card usage
  * 
  * @param	integer		card id
  * @param	integer		usage status
  * @return	object		ilFlashcardUsage
  */
 public function addCardUsage($a_card_id, $a_status = 0)
 {
     $usage = new ilFlashcardUsage($this->object->getId(), $this->user->getId(), $a_card_id);
     $usage->setStatus($a_status);
     $usage->save();
     $this->usages[$a_card_id] = $usage;
     return $usage;
 }