//contracts/GatewayVM.sol

    function checkBack(
        Machine memory vm,
        uint256 back
    ) internal pure returns (uint256) {
            if (back >= vm.stackSize) {
|>          uint256 index = vm.stackSize - 1 - back;
            revert InvalidStackIndex(index);
        }
        unchecked {
            return vm.stackSize - 1 - back;
        }
    }
