}
		line = after

		before, _, ok := strings.Cut(line, "|")
		if !ok {
			return nil, errors.New("invalid test data")
		}
		line = before

		hexBytes := strings.Fields(line)
		for _, hexByte := range hexBytes {
			val, err := strconv.ParseUint(hexByte, 16, 8)
			if err != nil {
				return nil, errors.New("invalid hex byte in test data: " + err.Error())
			}
			currentFlow = append(currentFlow, byte(val))
		}

		return 10 + digit - 'A', true
	}
	return -1, false
}

// hexByte returns the next hex-encoded (two-character) byte from the input.
// It returns ok==false if the next bytes in the input do not encode a hex byte.
// If the first byte is hex and the second is not, processing stops.
func (s *ss) hexByte() (b byte, ok bool) {
	rune1 := s.getRune()
	if rune1 == eof {
		return
	}

type hexBytes [2]byte

func (h *hexBytes) Scan(ss ScanState, verb rune) error {
	var b []byte
	_, err := Fscanf(ss, "%4x", &b)
	if err != nil {
		panic(err) // Really shouldn't happen.
	}
	copy((*h)[:], b)
	return err
}

func TestHexByte(t *testing.T) {
	var h hexBytes
	n, err := Sscanln("0123\n", &h)
	if err != nil {