src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<U> concatMapIterable(final Function<? super T, ? extends Iterable<? extends U>> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableFlattenIterable<T, U>(this, mapper));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<U> concatMapIterable(final Function<? super T, ? extends Iterable<? extends U>> mapper) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableFlattenIterable<T, U>(this, mapper));
    }
Returns an Observable that concatenate each item emitted by the source ObservableSource with the values in an
Iterable corresponding to that item that is generated by a selector.
<p>
<img width="640" height="275" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMapIterable.o.png" alt="">

<dl>
<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 ObservableSource
@param mapper
a function that returns an Iterable sequence of values for when given an item emitted by the
source ObservableSource
@return an Observable that emits the results of concatenating the items emitted by the source ObservableSource 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>