; Rees's cell (These 3 values are primitive in Scheme 48.)
(define (new-cell)(cons '() '()))
(define cell-ref car)
(define cell-set! set-car!)

;Rees's seal (verbatim from http://mumble.net/~jar/pubs/secureos/secureos.html)
(define (new-seal)
  (let ((instances (new-cell)))
    (cell-set! instances '())
    (let ((seal
           (lambda (rep)
             (let ((abs (new-cell)))
               (cell-set! instances
                          (cons (cons abs rep)
                                (cell-ref instances)))
              abs)))
         (unseal
          (lambda (abs)
            (let ((probe (assq abs (cell-ref instances))))
              (if probe
                  (cdr probe)
                  (error "invalid argument" abs)))))
        (sealed?
         (lambda (x)
           (if (assq x (cell-ref instances)) #t #f))))
     (list seal unseal sealed?))))
         
             

; From Seal to Amplification

; By example.
; css is a counter source source. (css) => (cs . amp) where
; cs is a counter source and amp is an amplifier for just the
; yields of cs.
; Each invocation of a counter source returns an independent counter control.
; If cc is a counter control then (cc) returns the value in the cell
; and then increments the counter.
; if amp came with cs then (amp (cs)) is a counter reseter for the
; cell that (cs) controls.
; A resetter can set the counter to anything it wants.

; To return multiple values:
(define (ylppa l p)(apply p l))


(define (css) (ylppa (new-seal)(lambda (seal unseal good?)
   (cons (lambda () (let ((cell (new-cell)))(cell-set! cell 0)
      (letrec ((me (lambda al
        (if (null? al)
          (let ((x (cell-ref cell)))(cell-set! cell (+ 1 x))x)
          (if (equal? al '("Who")) (seal (list me cell)))))))
         me)))
      (lambda (r)(ylppa (unseal (r "Who"))(lambda (id cell)(if (eq? r id)
         (lambda (nv)(cell-set! cell nv))))))))))

; minimal test
(define cp (css))
(define cs (car cp))
(define amp (cdr cp))
(define r1 (cs))
(define r2 (cs))
(define w1 (amp r1))
(w1 13)
(r1) ; expect 13