(use shift-reset)
(let ((captured-k
       (reset (print (shift f f)))))
  (print "str: '"
	 (with-output-to-string
	   (lambda ()
	     (captured-k "this should go to the string")))
	 "'"))

;; $ csi -qbn f-operator.scm 
;; this should go to the string
;; str: ''