(deffacts dane
(lista -2 3 -7 9 11 -20 19 6 -1)
(length 0)
)

(defrule dlugosc
?i<-(lista ?x $?reszta)
?ii<-(length ?ile)
=>
(retract ?i ?ii)
(assert (lista $?reszta))
(assert (length (+ ?ile 1)))
)