各位同學，剛搞懂這個。

(define call/cc call-with-current-continuation)
;Value: call/cc

(let ((x (call/cc (lambda (k) k))))
  (display '--)
  (display x)
  (x (lambda (ignore) "hi"))
  (display '>>)
  (display x))
--#[continuation 11]--#[compound-procedure 12]>>#[compound-procedure 12]
;Unspecified return value