public function add($loc, $lastmod = null, $changefreq = null, $priority = null)
    {
        $loc     = $this->escapeString($loc);
        $lastmod = !is_null($lastmod) ? $this->formatDate($lastmod) : null;

        return $this->addUrlToDocument(compact('loc', 'lastmod', 'changefreq', 'priority'));
    }