code: public static <L> ListenerList<L> addListener (ListenerList<L> list, L listener)
      {
          if (list == null) {
              list = new ListenerList<L>();
          }
          list.add(listener);
          return list;
      }
docs: Adds the supplied listener to the supplied list. If the list is null, a new listener list
  will be created. The supplied or newly created list as appropriate will be returned.