src/main/java/io/reactivex/Maybe.java
@BackpressureSupport(BackpressureKind.FULL)
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(MaybeSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return concat(this, other);
    }
@BackpressureSupport(BackpressureKind.FULL)
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(MaybeSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return concat(this, other);
    }
Returns a Flowable that emits the items emitted from the current MaybeSource, then the next, 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>Backpressure:</b></dt>
<dd>The operator honors backpressure from downstream.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param other
a MaybeSource to be concatenated after the current
@return a Flowable that emits items emitted by the two source MaybeSources, one after the other,
without interleaving them
@see <a href="http://reactivex.io/documentation/operators/concat.html">ReactiveX operators documentation: Concat</a>