% Problematic left-recursion example (requires tabling)

r(X, Y) :- r(X, Z), p(Z, Y).
r(X, Y) :- p(X, Y).
% r(a,d) works if goals reversed. Likely r(X, Z) will never succeed with Z = c
r(X, Y) :- r(X, Z), q(Z, Y).

p(a, b).
p(b, c).
q(c, d).

?- r(a,Y).