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