src/main/java/io/reactivex/parallel/ParallelFlowable.java
@BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    @CheckReturnValue
    @NonNull
    public final Flowable<T> sequential(int prefetch) {
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ParallelJoin<T>(this, prefetch, false));
    }
@BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    @CheckReturnValue
    @NonNull
    public final Flowable<T> sequential(int prefetch) {
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ParallelJoin<T>(this, prefetch, false));
    }
Merges the values from each 'rail' in a round-robin or same-order fashion and
exposes it as a regular Publisher sequence, running with a give prefetch value
for the rails.
<img width="640" height="602" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/parallelflowable.sequential.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator honors backpressure.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code sequential} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param prefetch the prefetch amount to use for each rail
@return the new Flowable instance
@see ParallelFlowable#sequential()
@see ParallelFlowable#sequentialDelayError(int)