code: public XSDocument startSchema(String targetNamespace) throws SAXException{
          xml.startElement(URI_XSD, "schema");
          if(targetNamespace!=null && !targetNamespace.isEmpty()){
              xml.declarePrefix(targetNamespace);
              xml.addAttribute("targetNamespace", targetNamespace);
          }
          return this;
      }
docs: <xsd:schema [targetNamespace="$targetNamespace"]>