#lang racket
(define x 0) ; shared state

(define p1-c #f)
(define p1
  (lambda ()
    ; set up a yield continuation
    (call/cc (lambda (yield) 
    (do () (#f) ; a do forever loop
           (set! x (+ x 1))
           (display x) (newline)
      
           (call/cc (lambda (c)  (set! p1-c c) (yield x)))
          
           )))
      )
    ; yield gets us here
)

; define the checkpoint continuations
(p1)
; what happens when we uncomment this
;(p1-c)