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% ===== Facts ===== father(ali, ahmad). father(ali, muhammad). father(ahmad, sara). father(ahmad, nora). father(ahmad, khan). father(muhammad, faisal). father(faisal, hassan). male(ali). male(ahmad). male(muhammad). male(khan). male(faisal). male(hassan). female(sara). female(nora). % ===== Rules ===== sister(X, Y) :- female(X), father(F, X), father(F, Y), X \= Y. brother(X, Y) :- male(X), father(F, X), father(F, Y), X \= Y. grandfather(G, C) :- father(G, P), father(P, C).