/**
  * parse search field from "Search" page
  * (required by view.php)
  */
 function parse_search_field()
 {
     if ($this->is_viewable) {
         if ($this->subfield) {
             return $this->subfield->parse_search_field();
         } else {
             return parent::parse_search_field();
         }
     } else {
         return '';
     }
 }