/**
  * display a form element for this field on the "Search" page
  *
  * @return HTML to send to browser
  */
 function display_search_field()
 {
     if ($this->is_viewable) {
         if ($this->subfield) {
             return $this->subfield->display_search_field();
         } else {
             return parent::display_search_field();
         }
     }
 }