code: public <T> ObservableTransformer<? super T, T> transform(Observer<? super T> observer) {
      return transform(observer, Utils.getObserverTag(observer));
    }
docs: Transforms an existing {@link Observable} by returning a new {@link Observable} that is
  automatically added to this {@link ObservableGroup}.
  <p> Convenience method
  for {@link #transform(Observer, String)} when {@code observer} only
  is subscribed to one {@link Observable}.