public static function joinTree($verbs) {
      $str = '';

      if (sizeof($verbs) > 0) {

        foreach ($verbs as $v) {
          $str .= self::openTag($v->getName(), $v->getAttributes());

          if ($v->hasVerbs()) {
            $str .= self::joinTree($v->getVerbs());    
          }

          $str .= $v->getText();


          $str .= self::closeTag($v->getName());
        }
      }

      return $str;
    }