code: func (h *Hasher) NewAsyncWriter(double bool) *AsyncHasher {
  	secsize := h.pool.SegmentSize
  	if double {
  		secsize *= 2
  	}
  	write := func(i int, section []byte, final bool) {
  		h.writeSection(i, section, double, final)
  	}
  	return &AsyncHasher{
  		Hasher:  h,
  		double:  double,
  		secsize: secsize,
  		write:   write,
  	}
  }
docs: // NewAsyncWriter extends Hasher with an interface for concurrent segment/section writes