Derived classes may also benefit from move assignment operations. If the
derived class and its base class support swapping then the implementation is
simple, following the standard shown earlier in section ref(MOVEASS). For the
class tt(Car) this could boil down to:
        verb(    Car &Car::operator=(Car &&tmp)
    {
        swap(tmp);
        return *this;
    })

If swapping is not supported then tt(std::move) can be used to call the
base class's move assignment operator:
        verb(    Car &Car::operator=(Car &&tmp)
    {
        static_cast<Land &>(*this) = std::move(tmp);
        // move Car's own data members next
        return *this;
    })

