modified SafeMath library example

SafeMath.sol

pragma solidity >= 0.4.25 < 0.6.0;

library SafeMath {

  struct Balance { uint256 _balance; }

  function add(Num storage self, uint256 amount) public {
    uint256 newBalance = self._ balance + amount;
    require(newBalance >= self._ balance, "addition overflow");
    self._ balance = newBalance;
  }

}