(deffacts dane
(rectangle 10 6)
(rectangle 7 5)
(rectangle 6 8)
(rectangle 2 5)
(sum 0)
)

(defrule sum-rectangle
(rectangle ?height ?width)
=>
(assert (area (* ?height ?width)))
)

(defrule sum-areas
?s<-(sum ?total)
?a<-(area ?n)
=>
(retract ?s ?a)
(assert (sum (+ ?total ?n)))
)