src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Flowable<U> concatMapIterable(Function<? super T, ? extends Iterable<? extends U>> mapper) {
        return concatMapIterable(mapper, 2);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Flowable<U> concatMapIterable(Function<? super T, ? extends Iterable<? extends U>> mapper) {
        return concatMapIterable(mapper, 2);
    }
Returns a Flowable that concatenate each item emitted by the source Publisher with the values in an
Iterable corresponding to that item that is generated by a selector.

<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 concatMapIterable} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <U>
the type of item emitted by the resulting Publisher
@param mapper
a function that returns an Iterable sequence of values for when given an item emitted by the
source Publisher
@return a Flowable that emits the results of concatenating 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>