(define (print-queue queue port)
  (define (display-elements ptr)
    (display (mcar ptr) port)
    (if (null? (mcdr ptr))
        #f
        (begin
          (display " " port)
          (display-elements (mcdr ptr)))))
  
  (if (empty-queue? queue)
      (display "()" port)
      (begin
        (display "{" port)
        (display-elements (front-ptr queue))
        (display "}" port))))