#lang racket

(define p1-c #f)

(define p1
  (lambda ()
    (call/cc (lambda (yield) ; yield control and return to caller
    (let ( (x 0) )
      
      (set! x (+ x 1))
      (display x) (newline)
      
      ; the first checkpoint
      (call/cc (lambda (c)  (set! p1-c c) (yield x)))
               
      (set! x (+ x 1))
      (display x) (newline)

      ; the second checkpoint
      (call/cc (lambda (c)  (set! p1-c c) (yield x)))
      
      ; there is no third checkpoint!
      (set! p1-c (lambda () "Hey, I'm finished"))
      
      "done"
      )))
    ))

(p1)

; what happens if you uncomment this
; (p1-c)