code: public <T> Observable<T> observable(Observer<? super T> observer) {
      return observable(observer, Utils.getObserverTag(observer));
    }
docs: Returns an existing {@link Observable} for the {@link TaggedObserver}.
  <p>
  <p> Does not change the locked status of this {@link ObservableGroup}.
  If it is unlocked, and the Observable has already emitted events,
  they will be immediately delivered. If it is locked then no events will be
  delivered until it is unlocked.