code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Completable flatMapCompletable(final Function<? super T, ? extends CompletableSource> mapper) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          return RxJavaPlugins.onAssembly(new MaybeFlatMapCompletable<T>(this, mapper));
      }
docs: Returns a {@link Completable} that completes based on applying a specified function to the item emitted by the
  source {@link Maybe}, where that function returns a {@link Completable}.
  <p>
  <img width="640" height="267" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Maybe.flatMapCompletable.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
  a function that, when applied to the item emitted by the source Maybe, returns a
  Completable
  @return the Completable returned from {@code mapper} when applied to the item emitted by the source Maybe
  @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>