[RxJS] Flatten a higher order observable with concatAll in RxJS

const clickObservable = Rx.Observable
  .fromEvent(document, 'click');

const clockObservable = clickObservable
  .map(click => Rx.Observable.interval(1000).take(5))
  .concatAll(); // the same as .mergeAll(1)

// flattening
// Observable<Observable<number>> ---> Observable<number>

/*
--------+--------------+-+----
        \        
         -0-1-2-3-4|
         
         concatAll
         
----------0-1-2-3-4-----0-1-2-3-4--0-1-2-3-4
*/

clockObservable
  .subscribe(x => console.log(x));