public function XSLT($filepath, $use_xslcache = true)
    {
        if ($use_xslcache && extension_loaded('xslcache')) {
            $xslt = new XSLTCache;
            $xslt->importStylesheet($filepath);
        } else {
            $xsl = new DOMDocument;
            $xsl->load($filepath);

            $xslt = new XSLTProcessor;
            $xslt->importStylesheet($xsl);
        }

        return $xslt->transformToXML(dom_import_simplexml($this));
    }