code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.FULL)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <U, V> Flowable<V> flatMapIterable(final Function<? super T, ? extends Iterable<? extends U>> mapper,
              final BiFunction<? super T, ? super U, ? extends V> resultSelector) {
          ObjectHelper.requireNonNull(mapper, "mapper is null");
          ObjectHelper.requireNonNull(resultSelector, "resultSelector is null");
          return flatMap(FlowableInternalHelper.flatMapIntoIterable(mapper), resultSelector, false, bufferSize(), bufferSize());
      }
docs: Returns a Flowable that emits the results of applying a function to the pair of values from the source
  Publisher and an Iterable corresponding to that item that is generated by a selector.
  <p>
  <img width="640" height="410" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMapIterable.f.r.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator honors backpressure from downstream and the source {@code Publisher}s is
  consumed in an unbounded manner (i.e., no backpressure is applied to it).</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code flatMapIterable} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param <U>
  the collection element type
  @param <V>
  the type of item emitted by the resulting Iterable
  @param mapper
  a function that returns an Iterable sequence of values for each item emitted by the source
  Publisher
  @param resultSelector
  a function that returns an item based on the item emitted by the source Publisher and the
  Iterable returned for that item by the {@code collectionSelector}
  @return a Flowable that emits the items returned by {@code resultSelector} for each item in the source
  Publisher
  @see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>