> So, #new: creates a point without values for x and y... What if I want  
> to only build objects that make sense, meaning that their instance  
> variables must conform to some invariant defined by the class?
>

you can't do this even in smalltalk.

...and checking arguments presence is not enough, you would need to
check its internal state too - and you shouldn't do that.



Best,
Kamil