src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable flatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return flatMapCompletable(mapper, false);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable flatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return flatMapCompletable(mapper, false);
    }
Maps each element of the upstream Observable into CompletableSources, subscribes to them and
waits until the upstream and all CompletableSources complete.
<p>
<img width="640" height="424" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMapCompletable.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code flatMapCompletable} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param mapper the function that received each source value and transforms them into CompletableSources.
@return the new Completable instance