(import (srfi 18))

(define m (make-mutex))
(define c (make-condition-variable))

(define countdown 5)

(define j
  (thread-start!
   (lambda ()
     (let loop ()
       (mutex-lock! m)
       (if (zero? countdown)
           (print "blastoff!")
           (begin
             (mutex-unlock! m c)
             (loop)))))))

(do ()
    ((zero? countdown))
  (print "blastoff in " countdown "...")
  (thread-sleep! 1)
  (set! countdown (sub1 countdown))
  (condition-variable-signal! c))

(thread-join! j)