text: import {Observable} from 'rxjs/Observable';
  import 'rxjs/add/observable/empty';
  import 'rxjs/add/operator/defaultIfEmpty';
  
  const source$ = Observable.empty();
  const new$ = source$.defaultIfEmpty('this is default');
  
  new$.subscribe(
    console.log,
    null,
    () => console.log('complete')
  );
  
  
  