% Lecture slide 4
male(albert).
female(alice).
male(edward).
female(victoria).
parent(albert,edward).
parent(victoria,edward).
parent(albert,alice).
parent(victoria,alice).

% Lecture slide 12
father(X,Y) :- male(X), parent(X,Y).
child_of(C,P) :- parent(P,C).

% Lecture slide 13
sibling(X,Y) :- parent(P,X), parent(P,Y).