src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    @BackpressureSupport(BackpressureKind.FULL)
    public static Completable concat(Publisher<? extends CompletableSource> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new CompletableConcat(sources, prefetch));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    @BackpressureSupport(BackpressureKind.FULL)
    public static Completable concat(Publisher<? extends CompletableSource> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new CompletableConcat(sources, prefetch));
    }
Returns a Completable which completes only when all sources complete, one after another.
<p>
<img width="640" height="237" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.concat.pn.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The returned {@code Completable} honors the backpressure of the downstream consumer
and expects the other {@code Publisher} to honor it as well.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concat} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param sources the sources to concatenate
@param prefetch the number of sources to prefetch from the sources
@return the Completable instance which completes only when all sources complete
@throws NullPointerException if sources is null