src/main/java/io/reactivex/Single.java
@BackpressureSupport(BackpressureKind.FULL)
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(SingleSource<? extends T> other) {
        return concat(this, other);
    }
@BackpressureSupport(BackpressureKind.FULL)
    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Flowable<T> concatWith(SingleSource<? extends T> other) {
        return concat(this, other);
    }
Returns a Flowable that emits the item emitted by the source Single, then the item emitted by the
specified Single.
<p>
<img width="640" height="335" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.concatWith.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The returned {@code Flowable} honors the backpressure of the downstream consumer.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param other
a Single to be concatenated after the current
@return a Flowable that emits the item emitted by the source Single, followed by the item emitted by
{@code t1}
@see <a href="http://reactivex.io/documentation/operators/concat.html">ReactiveX operators documentation: Concat</a>