import { Observable, Subject } from 'rxjs';
import { multiScan } from 'rxjs-multi-scan';
const initialCount: number = 0;
const add: Subject<number> = new Subject();
const subtract: Subject<number> = new Subject();
const count: Observable<number> = multiScan(
  add, (count, addend) => count + addend,
  subtract, (count, subtrahend) => count - subtrahend,
  initialCount);