private function combineList(
      $vars,
      $sep,
      $parameters,
      $combine,
      $reserved,
      $tag_empty,
      $combine_on_empty
  ) {
    $ret = array();
    foreach ($vars as $var) {
      $response = $this->combine(
          $var,
          $parameters,
          $sep,
          $combine,
          $reserved,
          $tag_empty,
          $combine_on_empty
      );
      if ($response === false) {
        continue;
      }
      $ret[] = $response;
    }
    return implode($sep, $ret);
  }