(unit/sig
  (mt mt? enq deq front-of put random-interval)
  (import plt:userspace^)
  
  ;; ------------------------------------------------------------------------
  ;; The Queue: 
  (define mt null)
  (define mt? null?)
  (define enq (lambda (aqueue element) (append aqueue (list element))))
  (define mk-front-of
    (lambda (tag msg op)
      (lambda (aqueue) (if (null? aqueue) (error tag msg) (op aqueue)))))
  (define deq (mk-front-of 'deq "empty queue" cdr))
  (define front-of (mk-front-of 'front-of "empty queue" car))
  (define put cons)

  ;; (random-interval low high)
  ;; returns a number from low...high inclusive
  ;;
  (define (random-interval low high)
    (+ low 
       (random (+ (- high low) 1))))
  )