/**
  * Add an object to the subject.
  * @param WEBCORE_OBJECT $obj
  */
 public function add_object($obj)
 {
     $this->num_objects += 1;
     $type_info = $obj->type_info();
     if (!isset($this->_items[$type_info->id])) {
         $item = new OBJECT_LIST_TITLE_ITEM();
         $item->type_info = $type_info;
         $this->_items[$type_info->id] = $item;
     }
     $this->_items[$type_info->id]->num_objects += 1;
 }