(define-unit bag-stack@
  (import)
  (export bag^)

  (define (bag? xs)
    (list? xs))

  (define (bag-empty? xs)
    (null? xs))

  (define empty-bag '())

  (define (bag-insert xs x)
    (cons x xs))

  (define (bag-peek xs)
    (car xs))

  (define (bag-remove xs)
    (cdr xs)))

-----------------------

(define-unit bag-fifo@
  (import)
  (export bag^)

  (define (bag? xs)
    (and (cons? xs)
         (list? (car xs))
         (list? (cdr xs))))

  (define (bag-empty? xs)
    (and (null? (car xs))
         (null? (cdr xs))))

  (define empty-bag (cons '() '()))

  (define (bag-insert xs x)
    (cons (cons x (car xs)) (cdr xs)))

  (define (bag-peek xs)
    (if (null? (cdr xs))
        (car (reverse (car xs)))
        (car (cdr xs))))

  (define (bag-remove xs)
    (if (null? (cdr xs))
        (cons '() (cdr (reverse (car xs))))
        (cons (car xs) (cdr (cdr xs)))))