public function obeyRobotsTxt($mode, $robots_txt_uri = null)
  {
    if (!is_bool($mode)) return false;
    
    $this->obey_robots_txt = $mode;
    
    if ($mode == true)
      $this->robots_txt_uri = $robots_txt_uri;
    else
      $this->robots_txt_uri = null;
    
    return true;
  }