library SafeMathMod {// Partial SafeMath Library

    function sub(uint256 a, uint256 b) internal pure returns (uint256 c) {
        require((c = a - b) < a);
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
        require((c = a + b) > a);
    }
    
    function mul(uint256 a, uint256 b) internal pure returns(uint c) {
    c = a * b;
    require(a == 0 || c / a == b);
  }
  
  function div(uint a, uint b) internal pure returns(uint c) {
    require(b > 0);
    c = a / b;
  }
}
function sub(uint256 a, uint256 b) internal pure returns (uint256 c) {
        require((c = a - b) < a);
    }
// Partial SafeMath Library