code: public int getIndex(XExtension extension) {
                  for (int i = 0; i < extensionList.size(); i++) {
                          if (extensionList.get(i).equals(extension)) {
                                  return i;
                          }
                  }
                  return -1;
          }
docs: Resolves the index of an extension, given that this extension has
  been previously registered with this manager instance. If the given
  index has not been registered previously, this method returns
  <code>-1</code>.
  
  @param extension The extension to look up the index for.
  @return Unique index of the requested extension (positive integer).