src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable flatMapCompletable(Function<? super T, ? extends CompletableSource> mapper, boolean delayErrors, int maxConcurrency) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
        return RxJavaPlugins.onAssembly(new FlowableFlatMapCompletableCompletable<T>(this, mapper, delayErrors, maxConcurrency));
    }
@CheckReturnValue
    @NonNull
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Completable flatMapCompletable(Function<? super T, ? extends CompletableSource> mapper, boolean delayErrors, int maxConcurrency) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
        return RxJavaPlugins.onAssembly(new FlowableFlatMapCompletableCompletable<T>(this, mapper, delayErrors, maxConcurrency));
    }
Maps each element of the upstream Flowable into CompletableSources, subscribes to them and
waits until the upstream and all CompletableSources complete, optionally delaying all errors.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>If {@code maxConcurrency == Integer.MAX_VALUE} the operator consumes the upstream in an unbounded manner.
Otherwise, the operator expects the upstream to honor backpressure. If the upstream doesn't support backpressure
the operator behaves as if {@code maxConcurrency == Integer.MAX_VALUE} was used.</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.
@param delayErrors if true errors from the upstream and inner CompletableSources are delayed until each of them
terminates.
@param maxConcurrency the maximum number of active subscriptions to the CompletableSources.
@return the new Completable instance