code: protected Element elementImpl(String name, String namespaceURI) {
          assertElementContainsNoOrWhitespaceOnlyTextNodes(this.xmlNode);
          if (namespaceURI == null) {
              return getDocument().createElement(name);
          } else {
              return getDocument().createElementNS(namespaceURI, name);
          }
      }
docs: Add a named and namespaced XML element to the document as a child of
  this builder's node.
  
  @param name
  the name of the XML element.
  @param namespaceURI
  a namespace URI
  @return
  
  @throws IllegalStateException
  if you attempt to add a child element to an XML node that already
  contains a text node value.