[Haskell-beginners] Enum for natural numbers

Hi, I have data Nat = Z | S Nat deriving (Eq,Ord,Show) toEnum should return Z for negative numbers. I did something like this but the problem is I don't know how to set "less than". I tried > and lt: instance Enum Nat where toEnum (gt 0) = S Z toEnum (lt 1) = Z -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01