func xtoi2(s string, e byte) (byte, bool) {
	if len(s) > 2 && s[2] != e {
		return 0, false
	}
	n, ei, ok := xtoi(s[:2], 0)
	return byte(n), ok && ei == 2
}
func xtoi2(s string, e byte) (byte, bool) {
	if len(s) > 2 && s[2] != e {
		return 0, false
	}
	n, ei, ok := xtoi(s[:2], 0)
	return byte(n), ok && ei == 2
}
// xtoi2 converts the next two hex digits of s into a byte.
// If s is longer than 2 bytes then the third byte must be e.
// If the first two bytes of s are not hex digits or the third byte
// does not match e, false is returned.