[Allow the user to use real numbers when playing with Nat
Joachim Breitner <mail@joachim-breitner.de>**20100904105316
 Ignore-this: f2cd7ccf0ee210badee7eb3bcac80b55
] hunk ./BUtil.hs 11
-data Nat = S Nat | Z deriving (Show,Eq)
+data Nat = S Nat | Z deriving (Eq)
+
+instance Show Nat where
+  show = show . fromNat
+
+instance Num Nat where
+  (+) = error "No operators defined for Nat"
+  (*) = error "No operators defined for Nat"
+  abs = error "No operators defined for Nat"
+  signum = error "No operators defined for Nat"
+  fromInteger n | n < 0  = error "Nat cannot be negative"
+                | n >= 0 = toNat n