data List = Cons Int List | Nil

data ListF a = Cons Int a | Nil

type ThreeList = ListF (ListF (ListF Void)))

ListF ListFF == ListFF

data Fix f = Fix {unfix :: f (Fix x)}
  type ListFF = Fix ListF

type ListAlg a = ListF a -> a

cata :: ListAlg a -> ListFF -> a
  cata f = f . fmap (cata f) . unfix

cata :: ListAlg a -> ListFF -> a
  cata :: (Either () (Int, a) -> a) -> ListFF -> a
  cata :: (() -> a) -> ((Int, a) -> a) -> ListFF -> a
  cata :: a -> (Int -> a -> a) -> ListFF -> a
  cata :: (Int -> a -> a) -> a -> [Int] -> a

cata :: (f a -> a) -> Fix f -> a