> But that never happens with mutators because you cannot
> HAVE any additional invariants. The derived type has
> to have exactly the same set of values as the base,
> in which case you cannot have any new interesting
> methods -- any method you have can always be defined
> in terms of the characteristic methods already in
> the base (assuming the whole public state is accessible
> of course).
>   
This is why you 1) define a base class which has a very loose set of 
invariants or 2) obscure state from inheriting classes.

~~ Robert.