//contracts/GatewayVM.sol

    function evalCommand(Machine memory vm, bytes[] memory outputs) internal view returns (uint8 /*exitCode*/) {
...
            } else if (op == GatewayOP.EVAL) {
                bool cond = !vm.isStackZeros(vm.pop());
                bytes memory program = vm.popAsBytes();
                if (cond) {
                    (uint256 pos, bytes memory buf) = (vm.pos, vm.buf); // save program
                    vm.buf = program;
                    vm.pos = 0;
|>                  uint8 exitCode = vm.evalCommand(outputs);
                    if (exitCode != 0) return exitCode;
                    (vm.pos, vm.buf) = (pos, buf); // restore program
                }
...
