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).