How to get the XML of a Batik SVGOMSymbolElement

I want to isolate an element from a svg file and get the code of this element (<svg><path/>...</svg>). I try with Batik, but I don't find how to transform a SVGOMSymbolElement into String.

String parser = XMLResourceDescriptor.getXMLParserClassName();
SAXSVGDocumentFactory f = new SAXSVGDocumentFactory(parser);
Document doc = f.createDocument("file.svg");
Element element = doc.getElementById("my-id");