public function _etag($id, $type = 'strong') {
        $id = (($type === 'weak') ? 'W/' : '').$id;

        $this->response()->header('ETag', $id);

        if (isset($_SERVER['HTTP_IF_NONE_MATCH']) &&
            $_SERVER['HTTP_IF_NONE_MATCH'] === $id) {
            $this->halt(304);
        }
    }