code: public StandaloneXmlParser addDelegate(QName elementName, XMLElementReader<List<ModelNode>> parser) {
          xmlMapper.registerRootElement(elementName, parser);
          return this;
      }
docs: Add a parser for a subpart of the XML model.
  
  @param elementName the FQ element name (i.e. subsystem name)
  @param parser      creates ModelNode's from XML input
  @return