src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> concatWith(@NonNull MaybeSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableConcatWithMaybe<T>(this, other));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> concatWith(@NonNull MaybeSource<? extends T> other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableConcatWithMaybe<T>(this, other));
    }
Returns an {@code Observable} that emits the items from this {@code Observable} 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>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 Observable completes normally.
@return the new Observable instance
@since 2.2