code: protected void namespaceImpl(String prefix, String namespaceURI) {
          if (! (this.xmlNode instanceof Element)) {
              throw new RuntimeException(
                  "Cannot add an attribute to non-Element underlying node: "
                  + this.xmlNode);
          }
          if (prefix != null && prefix.length() > 0) {
              ((Element) xmlNode).setAttributeNS("http://www.w3.org/2000/xmlns/",
                  "xmlns:" + prefix, namespaceURI);
          } else {
              ((Element) xmlNode).setAttributeNS("http://www.w3.org/2000/xmlns/",
                  "xmlns", namespaceURI);
          }
      }
docs: Add an XML namespace attribute to this builder's element node.
  
  @param prefix
  a prefix for the namespace URI within the document, may be null
  or empty in which case a default "xmlns" attribute is created.
  @param namespaceURI
  a namespace uri