code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.FULL)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <U> Flowable<U> flatMapIterable(final Function<? super T, ? extends Iterable<? extends U>> mapper, int bufferSize) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          ObjectHelper.verifyPositive(bufferSize, "bufferSize");
          return RxJavaPlugins.onAssembly(new FlowableFlattenIterable<T, U>(this, mapper, bufferSize));
      }
docs: Returns a Flowable that merges each item emitted by the source Publisher with the values in an
  Iterable corresponding to that item that is generated by a selector.
  <p>
  <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMapIterable.f.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator honors backpressure from downstream. The source {@code Publisher}s is
  expected to honor backpressure as well. If the source {@code Publisher} violates the rule, the operator will
  signal a {@code MissingBackpressureException}.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code flatMapIterable} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param <U>
  the type of item emitted by the resulting Iterable
  @param mapper
  a function that returns an Iterable sequence of values for when given an item emitted by the
  source Publisher
  @param bufferSize
  the number of elements to prefetch from the current Flowable
  @return a Flowable that emits the results of merging the items emitted by the source Publisher with
  the values in the Iterables corresponding to those items, as generated by {@code collectionSelector}
  @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>