code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static Completable complete() {
          return RxJavaPlugins.onAssembly(CompletableEmpty.INSTANCE);
      }
docs: Returns a Completable instance that completes immediately when subscribed to.
  <p>
  <img width="640" height="472" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.complete.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code complete} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @return a Completable instance that completes immediately