public static function unloadDocuments($id = null) 
    {
        if (isset($id)) {
            if ($id = self::getDocumentID($id)) {
                unset(phpQuery::$documents[$id]);
            }
        } else {
            foreach(phpQuery::$documents as $k => $v) {
                unset(phpQuery::$documents[$k]);
            }
        }
    }