Polymorphic catcalls

[Famous]

I think that one cannot expect the compiler to detect semantic
errors.

As far as I can see it the initial object oriented model is
unsound. Within the model 'BOY' and 'GIRL' are not valid
substitutes for 'SKIER' since they fail to satify the Liskov
substitution principle:

http://en.wikipedia.org/w/index.php?title=Liskov_substitution_principle&oldid=484745299

Cheers,
Famous