// appropriate length.
func formatPAXRecord(k, v string) (string, error) {
	if !validPAXRecord(k, v) {
		return "", ErrHeader
	}

	const padding = 3 // Extra padding for ' ', '=', and '\n'
	size := len(k) + len(v) + padding
	size += len(strconv.Itoa(size))
	record := strconv.Itoa(size) + " " + k + "=" + v + "\n"