src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable flatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return flatMapCompletable(mapper, false, Integer.MAX_VALUE);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable flatMapCompletable(Function<? super T, ? extends CompletableSource> mapper) {
        return flatMapCompletable(mapper, false, Integer.MAX_VALUE);
    }
Maps each element of the upstream Flowable into CompletableSources, subscribes to them and
waits until the upstream and all CompletableSources complete.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator consumes the upstream in an unbounded manner.</dd>
<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