interface Disposable {
  dispose(): void
}

interface AsyncObservable<T> {
  subscribe(observer: AsyncObserver<T>) : Disposable
  // OR
  forEach(observer: AsyncObserver<T>) : Disposable
}

interface AsyncObserver<T> {
  onNext(value: T) : Promise
  onError(error: Error) : Promise
  onCompleted() : Promise
}