public function generateVersioned($pathBase, $pathAppend)
    {
        $pathAbsolute = $pathBase . $pathAppend;
        if (!file_exists($pathAbsolute)) {
            throw new \Exception("cannot get cache busting path for file '$pathAbsolute'");
        }

        // get mod time
        $timeModified = filemtime($pathAbsolute);

        // return url to asset with modified time as query var
        return $this->generate() . $pathAppend . '?' . $timeModified;
    }