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

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

sex(X,female) :- female(X).
sex(X,male) :- male(X).

father(X) :- parent(X,Y),male(X).
mother(X) :- parent(X,Y),female(X).

son(X) :- parent(Y,X),male(X).
daughter(X) :- parent(Y,X),female(X).
	

sister(X, Y) :- parent(Z, X), parent(Z, Y),female(X), not(X = Y).

brother(X, Y) :- parent(Z, X), parent(Z, Y),male(X), not(X = Y).