%query: ackermann(i,i,o).
/* From Sterling and Shapiro; The Art of Prolog; Program 3.9
   

        ackermann(X,Y,A) :-
                A is the value of Ackermann's function for
                the natural numbers X and Y.
*/

        ackermann(0,N,s(N)).
        ackermann(s(M),0,Val) :- ackermann(M,s(0),Val).
        ackermann(s(M),s(N),Val) :-
                ackermann(s(M),N,Val1), ackermann(M,Val1,Val).