class Base {
public:
    
    class Thing {
    public:
        Thing(Base& b) : _b(b) {};
        
        Thing& operator+= (const Thing p) { _b.add(*this, p); return *this; };
        
        int k;
    protected:
        Base& _b;
    };

    void add(Thing &d, const Thing s) { d.k += s.k; }
};

template <class... Interfaces>
class Extensible : virtual public Base, virtual public Interfaces... {
    
    class Thing : virtual public Base::Thing, virtual public Interfaces::Thing... {

    };
};

class SomeInterface : Base {
    void multiply(Thing &d, const Thing s) { d.k *= s.k; }
    
    class Thing : public Base::Thing {
        Thing& operator*= (const Thing p) {
            //_b.multiply(*this, p); return *this; // <-- won't work of course
        };

    };

};

int main() {
    Extensible<SomeInterface> a;
    return 0;
}