作为我的反应式数据流中清理操作的一部分，我希望Observer在开始之前等待多个Observable的完成。
first_observable = rx.from_iterable(my_list).pipe(*actions)
second_observable = rx.just(somedata).pipe(*actionsfordata)
merge= rx.subject.Subject().pipe(merge_action)
merge.subscribe(on_completed = func1)
merge.subscribe(on_completed = func2)
merge.subscribe(on_completed = func3)
#WRONG! calls
#first_observable.subscribe(on_completed = merge.run)
#second_observable.subscribe(on_completed = merge.run)
# How to call merge when both observables finish?