% -*- prolog -*-

female(agica).
female(helena).
female(socha).
female(ziva).
male(barak).
male(fishel).
male(jacob).
parent(agica, helena).
parent(barak, socha).
parent(barak, ziva).
parent(fishel, barak).
parent(helena, socha).
parent(helena, ziva).
parent(jacob, fishel).

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

% this is wrong:
sibling(X, Y) :- parent(Z, X), parent(Z, Y).
