code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Completable concatWith(CompletableSource other) {
          ObjectHelper.requireNonNull(other, "other is null");
          return concatArray(this, other);
      }
docs: Concatenates this Completable with another Completable.
  <p>
  <img width="640" height="317" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.concatWith.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code concatWith} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param other the other Completable, not null
  @return the new Completable which subscribes to this and then the other Completable
  @throws NullPointerException if other is null
  @see #andThen(MaybeSource)
  @see #andThen(ObservableSource)
  @see #andThen(SingleSource)
  @see #andThen(Publisher)