(define (counter _)
  (let ((return identity))
    (let loop ((n 0))
      ;; yield
      (if (call/cc (lambda (k)
                     (set! counter
                       (lambda (outer)
                         (set! return outer)
                         (k #t)))
                     #f))
          (loop (add1 n)) ;; code executed at resume
          (return n)      ;; value returned by the yield
          ))))

;; to run it:
;; (call/cc counter)