public function export(core_kernel_classes_Class $source = null)
    {
        if ($source === null) {
            return core_kernel_api_ModelExporter::exportAll();
        }

        $graph = new EasyRdf_Graph();
        if ($source->isClass()) {
            $this->addClass($graph, $source);
        } else {
            $this->addResource($graph, $source);
        }
        $format = EasyRdf_Format::getFormat('rdfxml');
        return $graph->serialise($format);
    }