Tracked(T v): value(v), set(false) { }
Tracked &operator=(const Tracked &v) { if(v.set) { value = v.value; set = true; } return *this; }
Tracked(T v): value(v), set(false) { }
Tracked &operator=(const Tracked &v) { if(v.set) { value = v.value; set = true; } return *this; }