nil :: intlist
cons :: Int → intlist → intlist

fact :: Int → Int
fact(n) → 1                               | n ≤ 0
fact(n) → n * fact2(n - 1)                | n > 0

fact2 :: Int → Int
fact2(n) → fold([*], 1, genlist(n))

genlist :: Int → intlist
genlist(n) → nil                          | n ≤ 0
genlist(n) → cons(n, genlist(n-1))        | n > 0

fold :: (Int → Int → Int) → Int → intlist → Int
fold(f, y, nil) → y
fold(f, y, cons(x, l)) → f(x, fold(f, y, l))