private function getPreFiltersCacheId()
    {
        $cacheid = '';
        foreach($this->getPreFilters() as $key => $prefilter) {
            if ($prefilter instanceof PHPTAL_PreFilter) {
                $cacheid .= $key.$prefilter->getCacheId();
            } elseif ($prefilter instanceof PHPTAL_Filter) {
                $cacheid .= $key.get_class($prefilter);
            } else {
                $cacheid .= $key.$prefilter;
            }
        }
        return $cacheid;
    }