(define-signature bag^
  ((contracted
    [bag?       (-> any/c boolean?)]
    [empty-bag  (and/c bag? bag-empty?)]
    [bag-empty? (-> bag? boolean?)]
    [bag-insert (-> bag? any/c (and/c bag? (not/c bag-empty?)))]
    [bag-peek   (-> (and/c bag? (not/c bag-empty?)) any/c)]
    [bag-remove (-> (and/c bag? (not/c bag-empty?)) bag?)])))

;;STOS
(define-unit bag-stack@
  (import)
  (export bag^)
  ;;BAG?
  (define (bag? stack)
    (list? stack))

  ;;EMPTY
  (define (bag-empty? stack)
    (null? stack))

  ;;EMPTY-BAG
  (define empty-bag
    null)

  ;;PUSH
  (define (bag-insert stack element)
    (cons element stack))

  ;;POP
  (define (bag-remove stack)
    (if (bag-empty? stack)
        (error "stos jest pusty!")
        (cdr stack)))
 ;;PEEK
  (define (bag-peek stack)
    (car stack))
)