[Haskell-beginners] Enum for natural numbers

[kane96 at gmx.de]

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
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