return :: Int → ret
error :: ret

comp :: (Int → ret) → (Int → Int) → Int → ret
comp(f, g, x) → f(g(x))

fact :: Int → (Int → ret) → ret
fact(n, k) → error                        | n < 0
fact(n, k) → k(1)                         | n = 0
fact(n, k) → fact(n-1, comp(k, [*](n)))   | n > 0

/**
This is a case where the continuation arguably makes the program simpler; the first-order variation
would be:
  fact :: Int → ret
  helper :: Int → ret → ret
  
  fact(n) → error                         | n < 0
  fact(n) → return(1)                     | n = 0
  fact(n) → helper(n, fact(n-1))          | n > 0
  helper(n, return(k)) → return(n + k)
  helper(n, error) → error
*/