(deffacts empty-stack "Empty stack"
   (stack))

(defrule push-value "Push"
   ?push-value <- (push-value ?value)
   ?stack <- (stack $?rest)
   =>
   (retract ?push-value ?stack)
   (assert (stack ?value $?rest))
   (printout t "Pushed value " ?value crlf))

(defrule pop-value-valid "Pop (valid)"
   ?pop-value <- (pop-value)
   ?stack <- (stack ?value $?rest)
   =>
   (retract ?pop-value ?stack)
   (assert (stack $?rest))
   (printout t "Popped value " ?value crlf))

(defrule pop-value-invalid
   ?pop-value <- (pop-value)
   (stack)
   =>
   (retract ?pop-value)
   (printout t "Popping value from empty stack: ERROR" crlf))