src/main/java/io/reactivex/Observable.java
@SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concat(ObservableSource<? extends ObservableSource<? extends T>> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMap(sources, Functions.identity(), prefetch, ErrorMode.IMMEDIATE));
    }
@SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concat(ObservableSource<? extends ObservableSource<? extends T>> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMap(sources, Functions.identity(), prefetch, ErrorMode.IMMEDIATE));
    }
Returns an Observable that emits the items emitted by each of the ObservableSources emitted by the source
ObservableSource, one after the other, without interleaving them.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concat.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concat} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <T> the common element base type
@param sources
an ObservableSource that emits ObservableSources
@param prefetch
the number of ObservableSources to prefetch from the sources sequence.
@return an Observable that emits items all of the items emitted by the ObservableSources emitted by
{@code ObservableSources}, one after the other, without interleaving them
@see <a href="http://reactivex.io/documentation/operators/concat.html">ReactiveX operators documentation: Concat</a>