code: public <T> ObservableTransformer<? super T, T> transform(Observer<? super T> observer,
        String observableTag) {
      return new GroupSubscriptionTransformer<>(this, Utils.getObserverTag(observer),
          observableTag);
    }
docs: Transforms an existing {@link Observable} by returning a new {@link Observable} that is
  automatically added to this {@link ObservableGroup} with the provided {@code tag} when
  subscribed to.