code: func DecodeUint64(input string) (uint64, error) {
  	raw, err := checkNumber(input)
  	if err != nil {
  		return 0, err
  	}
  	dec, err := strconv.ParseUint(raw, 16, 64)
  	if err != nil {
  		err = mapError(err)
  	}
  	return dec, err
  }
docs: // DecodeUint64 decodes a hex string with 0x prefix as a quantity.