(import (rnrs)) ; for eof-object
(define (make-coroutine-generator proc)
  (define return #f)
  (define resume #f)
  (define yield (lambda (v) (call/cc (lambda (r) (set! resume r) (return v)))))
  (lambda () (call/cc (lambda (cc) (set! return cc)
                              (if resume
                                  (resume (if #f #f))  ; void? or yield again?
                                  (begin (proc yield)
                                         (set! resume (lambda (v) (return (eof-object))))
                                         (return (eof-object))))))))

 (define test (make-coroutine-generator
              (lambda (yield)
                (let loop ([i 0])
                  (when (< i 3)
                    (yield i)
                    (loop (+ i 1)))))))