protected function getDistinctURLHash(PHPCrawlerURLDescriptor $UrlDescriptor)
  {
    if ($this->url_distinct_property == self::URLHASH_URL)
      return md5($UrlDescriptor->url_rebuild);
    elseif ($this->url_distinct_property == self::URLHASH_RAWLINK)
      return md5($UrlDescriptor->link_raw);
    else
      return null;
  }