code: @BackpressureSupport(BackpressureKind.FULL)
      @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <R> Flowable<R> flatMapPublisher(Function<? super T, ? extends Publisher<? extends R>> mapper) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          return RxJavaPlugins.onAssembly(new MaybeFlatMapPublisher<T, R>(this, mapper));
      }
docs: Returns a Flowable that emits items based on applying a specified function to the item emitted by the
  source Maybe, where that function returns a Publisher.
  <p>
  <img width="640" height="260" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Maybe.flatMapPublisher.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The returned Flowable honors the downstream backpressure.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code flatMapPublisher} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param <R> the result value type
  @param mapper
  a function that, when applied to the item emitted by the source Maybe, returns a
  Flowable
  @return the Flowable returned from {@code func} 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>