code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <R> Single<R> flatMapSingle(final Function<? super T, ? extends SingleSource<? extends R>> mapper) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          return RxJavaPlugins.onAssembly(new MaybeFlatMapSingle<T, R>(this, mapper));
      }
docs: Returns a {@link Single} based on applying a specified function to the item emitted by the
  source {@link Maybe}, where that function returns a {@link Single}.
  When this Maybe completes a {@link NoSuchElementException} will be thrown.
  <p>
  <img width="640" height="356" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Maybe.flatMapSingle.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code flatMapSingle} 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
  Single
  @return the Single 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>