buy_stock(C) -< good_price(C).
~buy_stock(C) -< good_price(C), risky_company(C).
risky_company(C) -< in_fusion(C,D).
risky_company(C) -< closing(C).
~risky_company(C) -< in_fusion(C,D), strong(D).

good_price(acme).
in_fusion(acme,steel). % -< true.
strong(steel). % -< true.

% we could make this a built-in:
%true.
