(define yield (make-prompt-tag 'yield))

(call-with-prompt yield
  (λ ()
    (let iter ([lst (list 1)])
      (case
        [(null? lst) *unspecified*]
        [else
          (abort-to-prompt yield (car lst))
          (iter (cdr lst))])))
  (λ (k x)
     (simple-format #t "a) iterator yielded: ~a\n" x)
     (k)))