%Source: Random examples used for debugging termination analysis for arithmetic Prolog programs
% terminating
% Implements calculation of the Ackermann function.
% Direct implementation of the recursive definition by Péter (1935)
%query:ackermann(b,b,f).
ackermann(X, Y, Z) :- X =:= 0, Y >= 0, Z is Y + 1.
ackermann(X, Y, Z) :- X > 0, Y =:= 0,  ackermann(X - 1, 1, Z).
ackermann(X, Y, Z) :- X > 0, Y > 0, ackermann(X, Y - 1, W), ackermann(X - 1, W, Z).