;Rules
(defrule push-value
    ?push-value <- (push-value ?value)
    ?queue <- (queue $?rest)
    =>
    (retract ?push-value ?queue)
    (assert (queue $?rest ?value))
    (printout t "Pushing value: " ?value crlf))

(defrule pop-value-valid
    ?pop-value <- (pop-value)
    ?queue <- (queue ?value $?rest)
    =>
    (retract ?pop-value ?queue)
    (assert (queue $?rest))
    (printout t "Poping value: " ?value crlf))

(defrule pop-value-invalid
    ?pop-value <- (pop-value)
    (queue)
    =>
    (retract ?pop-value)
    (printout t "Error. Poping from empty queue!" crlf))

;Testing
(assert (queue))

(assert (push-value X))
(run)
(assert (push-value Y))
(run)
(assert (pop-value))
(run)
(assert (pop-value))
(run)
(assert (pop-value))
(run)