code: @Override
    public <R> StreamBuilderImpl<R,U> 
    reduce(R identity, 
           BiFunctionSync<R, ? super T, R> accumulator,
           BinaryOperatorSync<R> combiner)
    {
      return new ReduceAccumSync<>(this, identity, accumulator, combiner);
    }
docs: Reduce with an accumulator and combiner