code: public Xml addElement(Element element) {
          if(current != null) {
              // attach to existing model
              current.addChildElement(element);
          } else {
              // attach to a new model
              root =  element;
          }
          if(! element.isEmptyElementTag()) {
              current = element;
          }
          return this;
      }
docs: Add a new {@code Element} to this {@code Xml}
  @return {@code Xml} so that you can easily chain adds