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