text: import {Observable} from 'rxjs/Observable';
  import 'rxjs/add/observable/of';
  import 'rxjs/add/observable/interval';
  import 'rxjs/add/observable/never';
  import 'rxjs/add/operator/concat';
  import 'rxjs/add/operator/zipAll';
  import 'rxjs/add/operator/map';
  import 'rxjs/add/operator/take';
  
  const ho$ = Observable.interval(1000).take(2).concat(Observable.never())
    .map(x => Observable.interval(1500).map(y => x+':'+y).take(2));
  const concated$ = ho$.zipAll();
  
  concated$.subscribe(
    console.log,
    err => console.log('Error: ', err),
    () => console.log('complete')
  );
  
  