[Haskell-beginners] foldr for Nats

[kane96 at gmx.de]

Hello,
I have:
data Nat = Z | S Nat deriving (Eq,Ord,Show)
and should write a function that works for Nats like foldr for list where:
instance Enum Nat where
    toEnum i | i < 0 = error "foo"
                    | i == 0 = Z
                    | otherwise = S (toEnum (i-1))
Can someone give me a hint how to do this?

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