protected function get_unit_interaction(ImsXmlReader $item)
 {
     $interactions = $item->list_interactions();
     $main = self::get_main_interaction($item);
     if (count($interactions) == 2) {
         return $interactions[0]->responseIdentifier == $main->responseIdentifier ? $interactions[1] : $interactions[0];
     } else {
         return $item->get_default_result();
     }
 }