code: func asciiHexToByte(b byte) (byte, error) {
  	switch {
  	case b >= '0' && b <= '9':
  		return b - '0', nil
  	case b >= 'a' && b <= 'f':
  		return b - 'a' + 10, nil
  	default:
  		return 0, ErrInvalidPktLen
  	}
  }
docs: // turns the hexadecimal ascii representation of a byte into its
  // numerical value.  Example: from 'b' to 11 (0xb).