public function isChildBookmark($bookmarkIndex)
    {

        if ($bookmarkIndex === '')
            throw new Exception('bookmark index not specified');
        $strURI = Product::$baseProductUri . '/pdf/' . $this->getFileName() . '/bookmarks/' . $bookmarkIndex;
        $signedURI = Utils::sign($strURI);
        $responseStream = Utils::processCommand($signedURI, 'GET', '', '');
        $json = json_decode($responseStream);
        return $json->Bookmark;
    }