我有以下代碼（RxJava 1.3.8）：Scheduler scheduler = Schedulers.from(Executors.newSingleThreadExecutor());Subscription subscription1 = Completable.fromCallable(() -> {  Thread.sleep(1000);  System.out.println("first Callable executed");  return 0;})    .subscribeOn(scheduler)    .subscribe();Subscription subscription2 = Completable.fromCallable(() -> {  Thread.sleep(1000);  System.out.println("second Callable executed");  return 0;})    .subscribeOn(scheduler)    .subscribe();CompositeSubscription subscriptions = new CompositeSubscription();subscriptions.addAll(subscription1, subscription2);subscriptions.clear();輸出：第一個 Callable 執行第二個 Callable 執行問題是 - 為什么第二個 Callable 被執行？在它運行之前，如果取消訂閱，我希望訂閱檢查并取消執行。