   function add512(uint256 addendA0, uint256 addendA1, uint256 addendB0, uint256 addendB1) public pure returns (uint256 sum0, uint256 sum1) {
        assembly {
            sum0 := add(addendA0, addendB0)
            carry  := lt(sum0, addendA0)
            sum1 := add(add(addendA1, addendB1), carry)
        }
        if (addendA1 > sum1 ||      ((sum1 == addendA1 || sum1 == addendB1) && (carry < addendA0 || carry < addendB0))
) revert AddOverflow(addendA0, addendA1, addendB0, addendB1);
// 
    }
