public function getRdfString($instances)
    {
        $api = core_kernel_impl_ApiModelOO::singleton();
        $rdf = '';
        $xmls = [];
        foreach ($instances as $instance) {
            $xmls[] = $api->getResourceDescriptionXML($instance->getUri());
        }

        if (count($xmls) === 1) {
            $rdf = $xmls[0];
        } elseif (count($xmls) > 1) {

            $baseDom = new DomDocument();
            $baseDom->formatOutput = true;
            $baseDom->loadXML($xmls[0]);

            for ($i = 1, $iMax = count($xmls); $i < $iMax; $i++) {

                $xmlDoc = new SimpleXMLElement($xmls[$i]);
                foreach ($xmlDoc->getNamespaces() as $nsName => $nsUri) {
                    if (!$baseDom->documentElement->hasAttribute('xmlns:' . $nsName)) {
                        $baseDom->documentElement->setAttribute('xmlns:' . $nsName, $nsUri);
                    }
                }
                $newDom = new DOMDocument();
                $newDom->loadXML($xmls[$i]);
                foreach ($newDom->getElementsByTagNameNS('http://www.w3.org/1999/02/22-rdf-syntax-ns#', 'Description') as $desc) {
                    $newNode = $baseDom->importNode($desc, true);
                    $baseDom->documentElement->appendChild($newNode);
                }
            }

            $rdf = $baseDom->saveXML();
        }

        return $rdf;
    }