private function _set_session($attr){
    if ( $this->session->has(self::$sn) ){
      $args = \func_get_args();
      if ( (\count($args) === 2) && \is_string($args[0]) ){
        $attr = [$args[0] => $args[1]];
      }
      foreach ( $attr as $key => $val ){
        if ( \is_string($key) ){
          $this->session->set($val, self::$sn, $key);
        }
      }
    }
    return $this;
  }