const render$ = new Subject()

render$.subscribe(
  value => output.textContent = '' + value
);

// Just a source that increments a number over time when
// you click buttons.
const source$ = render$.pipe(
  startWith(INITIAL_VALUE),
  switchMap(initialValue => (
    merge(
      // when we click "+ 1" it adds `1`
      add1Click$.pipe(map(() => 1)),
      // when we click "+ 2" it adds `2`
      add2Click$.pipe(map(() => 2)),
    )
    .pipe(
      // A reducer to accumlate our number
      scan((acc, n) => acc + n, initialValue || 0),
    )
  )),
  // Start with 0, so we have an initial value to display
  startWith(INITIAL_VALUE),
);

// Here we're adding our custom `undo` and subscribing to it.
source$.pipe(
  // "Undo" whenever we click the "Undo" button.
  undo(undoClick$, redoClick$),
).subscribe(render$);