Обратен извод (backward chaining)
if gives_milk(X) then mammal(X).
if has_feathers(X) and lays_eggs(X)then bird(X).
if has_gills(X) and lays_eggs(X) then fish(X).
if bird(X) and not penguin(X)
fact(has_feathers(tweety)).
bird(tweety) and not penguin(tweety)