; This program calculates the average area of some rectangles

(deffacts initial-information
(rectangle 10 6)
(rectangle 7 5)
(rectangle 6 8)
(rectangle 2 5)
(rectangle 9 4)
(sum 0)
(count 0))

(defrule sum-rectangles
  (declare (salience 30))
  (rectangle ?height ?width)
=>
  (assert (add-to-sum =(* ?height ?width))))

(defrule sum-areas
  (declare (salience 20))
  ?sum-adr<-(sum ?total)
  ?new-area-adr<-(add-to-sum ?area)
  ?count-adr<-(count ?counter)
=>
  (retract ?sum-adr  ?new-area-adr ?count-adr)
  (assert (sum =(+ ?total ?area)))
  (assert (count =(+ ?counter 1))))

(defrule average 
  (declare (salience 10))
  ?sum-adr<-(sum ?total)
  ?count-adr<-(count ?counter)
=>
  (printout t crlf "Here is the average area " (/ ?total ?counter) crlf crlf))