code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static Completable amb(final Iterable<? extends CompletableSource> sources) {
          ObjectHelper.requireNonNull(sources, "sources is null");
  
          return RxJavaPlugins.onAssembly(new CompletableAmb(null, sources));
      }
docs: Returns a Completable which terminates as soon as one of the source Completables
  terminates (normally or with an error) and disposes all other Completables.
  <p>
  <img width="640" height="518" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.amb.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code amb} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param sources the array of source Completables. A subscription to each source will
  occur in the same order as in this Iterable.
  @return the new Completable instance
  @throws NullPointerException if sources is null