library SafeMath {//安全数值运算库
        function mul(uint256 a, uint256 b) internal pure returns (uint256) {
            if (a == 0) {
                return 0; 
            }
            uint256 c = a * b;
            assert(c / a == b);
            return c; 
        }
        function div(uint256 a, uint256 b) internal pure returns (uint256) {
// assert(b > 0); // Solidity automatically throws when dividing by 0
            uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold
            return c; 
        }

        function sub(uint256 a, uint256 b) internal pure returns (uint256) {
            assert(b <= a);
            return a - b; 
        }
        function add(uint256 a, uint256 b) internal pure returns (uint256) {
            uint256 c = a + b;
            assert(c >= a);
            return c; 
        }
    }
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
        return 0; 
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c; 
}

//安全数值运算库