Implicit in stretching one axis and not the other axis of an object is that only the one axis gets stretched.

The contract isn't just the one specified by the type signature; it's what follows the API specification _and_ the principle of least surprise.

After all, otherwise you could correctly have the width and height scaling methods always resize the other, too, even for Rectangle! Then it would work for Square. But that would be pointless--you should just have a single `scale` method if you want that API.