%   Evan Tick (from Lisp version by R. P. Gabriel)
%
%   (almost) Takeuchi function (recursive arithmetic)

tak(R) :- tak(18, 12, 6, R).

tak(X, Y, Z, A) :-
    {
      X =< Y,
      Z = A
    }.

tak(X, Y, Z, A) :-
    {
      X >= Y+1
    },
    tak(X-1, Y,   Z,  A1),
    tak(Y-1, Z,   X,  A2),
    tak(Z-1, X,   Y,  A3),
    tak(A1,  A2,  A3, A ).