fact: Definition ack (n : nat) := (fix F (n0 : nat) : nat -> nat := match n0 with | O => S | S n1 => fun m : nat => (fix F0 (n2 : nat) : nat := match n2 with | O => F n1 1 | S n3 => F n1 (F0 n3) end) m end) n.