/* Let's assert some facts */

father(john, jane).

mother(jane, sue).

mother(jane, liz).

mother(jane, jeff).


/* Let's assert some rules */

parent(X,Y) :- mother(X,Y).

parent(X,Y) :- father(X,Y).


/* How about grandparents? */

grandparent(X,Y) :-
	parent(X,Z),
	parent(Z,Y).


grandfather(X,Y) :-
	father(X,Z),
	parent(Z,Y).

grandmother(X,Y) :-
	mother(X,Z),
	parent(Z,Y).


/* Siblings */

sibling(X,Y) :-
	parent(Z,X),
	parent(Z,Y),
	X \= Y.