        /// concatenated symbol string
        tokenSymbol = string(
            abi.encodePacked(
                "ROLLA",
                "-",
                underlying,
                "-",
                _uintToChars(day),
                monthSymbol,
                _uintToChars(year),
                "-",
                displayStrikePrice,
                "-",
                typeSymbol
            )
        );
    /// @return 2 characters that correspond to a number
    function _uintToChars(uint256 _number)
        internal
        pure
        virtual
        returns (string memory)
    {
        if (_number > 99) {
            _number %= 100;
        }

        string memory str = Strings.toString(_number);

        if (_number < 10) {
            return string(abi.encodePacked("0", str));
        }

        return str;
    }
