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function add(uint256 a, uint256 b) internal pure returns (uint256);
function sub(uint256 a, uint256 b) internal pure returns (uint256);
function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256);
function mul(uint256 a, uint256 b) internal pure returns (uint256);
function div(uint256 a, uint256 b) internal pure returns (uint256);
function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256);
function mod(uint256 a, uint256 b) internal pure returns (uint256);
function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256);
function sqrrt(uint256 a) internal pure returns (uint256 c);