Now I hold an `Observable<Observable<Integer>`, how can I transfer it into `Observable<int[]>` that contains the n-ary Cartesian product?

For example:

```
Observable<Observable<Integer> ob = Observable.just(
  Observable.just(0,1),
  Observable.just(2,3),
  Observable.just(4,5)
  );
ob...... ->   (0,2,4), (0,3,4), (0,2,5), (0,3,5), (1,2,4), (1,3,4), (1,2,5), (1,3,5)

```