ack(M, N, R) :-
   { M = 0, R = N+1 }.
ack(M, N, R) :-
   { M >= 1, N = 0, M = M1+1 }, ack(M1, 1, R).
ack(M, N, R) :-
   { M >= 1, N >= 1, M = M1+1, N = N1+1 }, ack(M, N1, T), ack(M1, T, R).