code: @SuppressWarnings("unchecked")
    <C, T> Stage<T> doCollectEmpty(
        final Function<? super Collection<C>, ? extends T> collector
    ) {
      try {
        return this.completed(collector.apply((Collection<C>) EMPTY_RESULTS));
      } catch (Exception e) {
        return failed(e);
      }
    }
docs: Shortcut for when the list of stages is empty.
  
  @param collector collector to apply