#!/usr/bin/env runhaskell
newtype Mu a = Roll (Mu a -> (a -> a))
unroll (Roll x) = x

-- y4h :: (a -> a) -> a -> a
y4h = \f -> (\x z -> f ((unroll x) x z))
        (Roll (\x z -> f ((unroll x) x z)))

-- _fact :: (Int -> Int) -> Int -> Int
_fact f x = if x <= 0
               then 1
               else x * (f (x-1))

-- fact :: Int -> Int
fact = y4h  _fact undefined

main = print $ fact 10