template<class T>
class AorB {
    T * a;
    T * b;
    bool flipbit = true;
public:
    AorB(T * first, T * second) : a(first), b(second)
    {
    }
    T * current() {
        return flipbit ? a : b;
    }
    T * next() {
        return flipbit ? b : a;
    }
    T * flip() {
        flipbit = !flipbit;
        return current();
    }
    T & operator[](const int index) {
        return *(current() + index);
    }
};