code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Completable andThen(CompletableSource next) {
          return concatWith(next);
      }
docs: Returns a Completable that first runs this Completable
  and then the other completable.
  <p>
  <img width="640" height="437" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.andThen.c.png" alt="">
  <p>
  This is an alias for {@link #concatWith(CompletableSource)}.
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code andThen} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param next the other Completable, not null
  @return the new Completable instance
  @throws NullPointerException if other is null