(define run-me-a #f)
(define run-me-b #f)

(display "a\n")

(call-with-current-continuation
 (lambda (cont)
   (set! run-me-a cont)
   (call-with-current-continuation
    (lambda (cont)
      (set! run-me-b cont)
      (run-me-a #t)))))

(display "b\n")

(run-me-b #t)

(display "c\n")