(let ackermann (fun (m n) {
  (if (> m 0)
    (if (= 0 n)
      (ackermann (- m 1) 1)
      (ackermann (- m 1) (ackermann m (- n 1))))
    (+ 1 n)) }))

(assert (= 509 (ackermann 3 6)) "(ackermann 3 6) == 509")