code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      @BackpressureSupport(BackpressureKind.FULL)
      private static Completable merge0(Publisher<? extends CompletableSource> sources, int maxConcurrency, boolean delayErrors) {
          ObjectHelper.requireNonNull(sources, "sources is null");
          ObjectHelper.verifyPositive(maxConcurrency, "maxConcurrency");
          return RxJavaPlugins.onAssembly(new CompletableMerge(sources, maxConcurrency, delayErrors));
      }
docs: Returns a Completable instance that keeps subscriptions to a limited number of sources at once and
  completes only when all source Completables terminate in one way or another, combining any exceptions
  thrown by either the sources Observable or the inner Completable instances.
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The returned {@code Flowable} honors the backpressure of the downstream consumer
  and expects the other {@code Publisher} to honor it as well.
  <dt><b>Scheduler:</b></dt>
  <dd>{@code merge0} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param sources the iterable sequence of sources.
  @param maxConcurrency the maximum number of concurrent subscriptions
  @param delayErrors delay all errors from the main source and from the inner Completables?
  @return the new Completable instance
  @throws NullPointerException if sources is null
  @throws IllegalArgumentException if maxConcurrency is less than 1