code: func (p *hashed) decodeCost(sbytes []byte) (int, error) {
  	cost, err := strconv.Atoi(string(sbytes[0:2]))
  	if err != nil {
  		return -1, err
  	}
  	err = checkCost(cost)
  	if err != nil {
  		return -1, err
  	}
  	p.cost = cost
  	return 3, nil
  }
docs: // sbytes should begin where decodeVersion left off.