var valid: bool = true;

                        valid = !pred.?.marked and (succ == null or !succ.?.marked) and

                        const valid = !pred.?.marked and (succ == null or !succ.?.marked) and

                        var valid = true;

                            valid = !pred.?.marked and pred.?.nexts[level] == succ;

                            const valid = !pred.?.marked and pred.?.nexts[level] == succ;