code: public void register(XExtension extension) {
                  extensionMap.put(extension.getUri(), extension);
                  // replace the registered index in the list with the new extension.
                  int i = extensionList.indexOf(extension);
                  if (i < 0) {
                          extensionList.add(extension);
                  } else {
                          extensionList.remove(i);
                          extensionList.add(i, extension);
                  }
          }
docs: Explicitly registers an extension instance with the extension
  manager.
  
  @param extension The extension to be registered.