code: public static <T> Transformer<Observable<T>, T> flatten() {
          return new Transformer<Observable<T>, T>() {
  
              @Override
              public Observable<T> call(Observable<Observable<T>> source) {
                  return source.flatMap(Functions.<Observable<T>> identity());
              }
          };
      }
docs: Returns an {@link Transformer} that flattens a sequence of
  {@link Observable} into a flat sequence of the items from the
  Observables. This operator may interleave the items asynchronously.
  
  @return Transformer