Just to be sure. In the following code, the returned mark set contains the
key-value pair {key, mark1} but not {key, mark2}, correct?
===
(let ((marks #f))
(call/cc
(lambda (cont)
(with-continuation-marks 'key 'mark1
(begin
(set! marks (continuation-marks cont))
(with-continuation-marks 'key 'mark2 #f)))))
marks)
===
