rxgroups/src/main/java/com/airbnb/rxgroups/ObservableGroup.java
public <T> ObservableTransformer<? super T, T> transform(Observer<? super T> observer,
      String observableTag) {
    return new GroupSubscriptionTransformer<>(this, Utils.getObserverTag(observer),
        observableTag);
  }
public <T> ObservableTransformer<? super T, T> transform(Observer<? super T> observer,
      String observableTag) {
    return new GroupSubscriptionTransformer<>(this, Utils.getObserverTag(observer),
        observableTag);
  }
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.