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 {
		t.Fatal(err)
	}
	if n != 1 {
		t.Fatalf("expected 1 item; scanned %d", n)
	}