code: public void registerExtension(String namespace)
    {
      if (namespaceIndex(namespace, m_extensions) == -1)
      {
        int predef = namespaceIndex(namespace, m_predefExtensions);
        if (predef !=-1)
          m_extensions.add(m_predefExtensions.get(predef));
        else if (!(m_unregisteredExtensions.contains(namespace)))
          m_unregisteredExtensions.add(namespace);       
      }
    }
docs: If necessary, register the extension namespace found compiling a function or
  creating an extension element.
  
  If it is a predefined namespace, create a
  support object to simplify the instantiate of an appropriate ExtensionHandler
  during transformation runtime. Otherwise, add the namespace, if necessary,
  to a vector of undefined extension namespaces, to be defined later.