private function prepareForSerialization($serializablePage, $format, $entries)
    {
        $engine     = $serializablePage->getEngine();
        $keyword    = $serializablePage->getKeyword();
        $pageUrl    = $serializablePage->getPageUrl();
        $pageNumber = $serializablePage->getPageNumber();
        $age        = $serializablePage->getAge();
        return self::getFormatClassName($format, array($engine, $pageNumber, $pageUrl,
                                        $keyword, $age, $entries));
    }