src/main/java/io/reactivex/parallel/ParallelFlowable.java
@BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    @CheckReturnValue
    @NonNull
    public final Flowable<T> sequentialDelayError() {
        return sequentialDelayError(Flowable.bufferSize());
    }
@BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    @CheckReturnValue
    @NonNull
    public final Flowable<T> sequentialDelayError() {
        return sequentialDelayError(Flowable.bufferSize());
    }
Merges the values from each 'rail' in a round-robin or same-order fashion and
exposes it as a regular Flowable sequence, running with a default prefetch value
for the rails and delaying errors from all rails till all terminate.
<p>
This operator uses the default prefetch size returned by {@code Flowable.bufferSize()}.
<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 sequentialDelayError} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.0.7 - experimental
@return the new Flowable instance
@see ParallelFlowable#sequentialDelayError(int)
@see ParallelFlowable#sequential()
@since 2.2