src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMap(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
        return concatMap(mapper, 2);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMap(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
        return concatMap(mapper, 2);
    }
Returns a new Observable that emits items resulting from applying a function that you supply to each item
emitted by the source ObservableSource, where that function returns an ObservableSource, and then emitting the items
that result from concatenating those resulting ObservableSources.
<p>
<img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMap.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatMap} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <R> the type of the inner ObservableSource sources and thus the output type
@param mapper
a function that, when applied to an item emitted by the source ObservableSource, returns an
ObservableSource
@return an Observable that emits the result of applying the transformation function to each item emitted
by the source ObservableSource and concatenating the ObservableSources obtained from this transformation
@see <a href="http://reactivex.io/documentation/operators/flatmap.html">ReactiveX operators documentation: FlatMap</a>