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12 lines
209 B
Idris
12 lines
209 B
Idris
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data IsS : Nat -> Type where
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S : (n : Nat) -> IsS (S n)
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isS : (n : Nat) -> Maybe (IsS n)
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isS (S n) = Just (S n)
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isS _ = Nothing
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pred : Nat -> Nat
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pred n with (isS n)
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_ | Nothing = Z
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_ | Just (S n) = n
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