% Oblog program: simple family relationships
class person(name).

class family.
  method parent(person(P), person(C)) :-
    P = 'John', C = 'Mary'.
  method parent(person(P), person(C)) :-
    P = 'John', C = 'Tom'.
  method parent(person(P), person(C)) :-
    P = 'Mary', C = 'Ann'.

  method grandparent(person(GP), person(GC)) :-
    self.parent(GP, P),
    self.parent(P, GC).
end.

% Query: who is Ann's grandparent?
:- family F, F.grandparent(GP, person('Ann')), write(GP).