predicates can_buy(symbol, symbol) person(symbol) car(symbol) likes(symbol, symbol) for_sale(symbol) clauses can_buy(X, Y) :- person(X), car(Y), likes(X, Y), for_sale(Y). person(kelly). person(judy). car(mercedes). car(benz). likes(kelly, benz). likes(judy, pizza). for_sale(pizza). for_sale(mercedes). for_sale(benz).