[plt-scheme] value-based event?

Hi all -

is it possible to create an event based on whether a variable has value - if
it is #f, then the event blocks, and when the event gets set to something
else other than #f, the event unblocks?

Something like:

(define x #f)
(thread (lambda ()
          (sleep 10)
          (set! x 1)))

(sync (has-value-evt x)) ;; unblock when (set! x 1) is reached...

Thanks,
yc
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