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fact 0 = 1
fact 1 = 1
fact n | n < 0 = error "Negative factorial!"
       | otherwise = n * fact (n - 1)

fact 0 = 1
fact 1 = 1
...
       | otherwise = g 1 n where
            g x 1 = x
            g x n' = g (x * n') (n' - 1)