code: protected String lookupNamespaceURIImpl(String name) {
          String prefix = getPrefixFromQualifiedName(name);
          String namespaceURI = this.xmlNode.lookupNamespaceURI(prefix);
          return namespaceURI;
      }
docs: Look up the namespace matching the current builder node's qualified
  name prefix (if any) or the document's default namespace.
  
  @param name
  the name of the XML element.
  
  @return
  The namespace URI, or null if none applies.