lock: std.Thread.Mutex,

            lock: std.Thread.Mutex = .{},

                var n = try allocator.create(Node);
                n.key = key;
                n.value = value;
                n.lock = std.Thread.Mutex{};
                n.nexts = try allocator.alloc(?*Node, level + 1); // level can be 0

                const n = try allocator.create(Node);
                n.* = Node{
                    .key = key,
                    .value = value,
                    .nexts = try allocator.alloc(?*Node, level + 1), // level can be 0
                };