src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(@NonNull MaybeSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new FlowableConcatWithMaybe<T>(this, other));
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(@NonNull MaybeSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new FlowableConcatWithMaybe<T>(this, other));
    }
Returns a {@code Flowable} that emits the items from this {@code Flowable} followed by the success item or terminal events
of the other {@link MaybeSource}.
<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 supports backpressure and makes sure the success item of the other {@code MaybeSource}
is only emitted when there is a demand for it.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.10 - experimental
@param other the MaybeSource whose signal should be emitted after this Flowable completes normally.
@return the new Flowable instance
@since 2.2