code: public void registerExtension(ExtensionNamespaceSupport extNsSpt)
    {
      String namespace = extNsSpt.getNamespace();
      if (namespaceIndex(namespace, m_extensions) == -1)
      {
        m_extensions.add(extNsSpt);
        if (m_unregisteredExtensions.contains(namespace))
          m_unregisteredExtensions.remove(namespace);
      }
      
    }
docs: Register the extension namespace for an ElemExtensionDecl or ElemFunction,
  and prepare a support object to launch the appropriate ExtensionHandler at
  transformation runtime.