code: 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
  }
docs: // 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.