[contest] Is a Cow an animal ++

[bearophile]

Philippe Sigaud:
> Encoding the cow's state in its type is no good either, as you cannot change
> a cow's type. I think OCaml does it with algebraic data types. These are
> doable in D, but only testable at runtime...

More answers later. In the meantime this contains something about the "typestate":
http://d.puremagic.com/issues/show_bug.cgi?id=4571

Bye,
bearophile