peters-buy() :- buy(peters,What) & What.

buy(Broker,Prod) :- want(customer(Broker),Prod),
                    deal(Prod,cost(Prod),benefit(Prod)).

deal(Prod,Cost,Benefit) :- >=(Benefit,Cost).

want(miller,stock1).
want(miller,stock2).
want(fuller,stock3).

customer(peters) :& miller.
customer(peters) :& fuller.

cost(stock1) :& 165.
cost(stock2) :& 130.
cost(stock3) :& 175.

benefit(stock1) :& 160.
benefit(stock2) :& 130.
benefit(stock3) :& 180.