code: func sum(s ...string) (int64, error) {
  	var total int64
  	for _, v := range s {
  		i, err := strconv.ParseInt(v, 10, 64)
  		if err != nil {
  			return 0, err
  		}
  		total += i
  	}
  	return total, nil
  }
docs: // sum will convert the list of strings into int64s and total them together.