% Run as
% $ swipl -g "main,halt." grandparent.pl

% main/0, mother/2, and father/2 was added by hakan:
main :-
    grandparent(X,Y),
    format("~w is a grandparent of ~w~n",[X,Y]),
    fail.

mother(ann,amy).
mother(ann,andy).
mother(amy,amelia).
mother(linda,gavin).
father(steve,amy).
father(steve,andy).
father(gavin,amelia).
father(andy,spongebob).


grandparent(X, Y) :- mother(X, Z), mother(Z, Y).
grandparent(X, Y) :- mother(X, Z), father(Z, Y).
grandparent(X, Y) :- father(X, Z), mother(Z, Y).
grandparent(X, Y) :- father(X, Z), father(Z, Y).