- M-x run-scheme 
- choose csi

eval this:

(use srfi-18)
(thread-start! (lambda ()
                 (let loop ([n 0])
                   (thread-sleep! 1)
                   (print "ping " n)
                   (loop (add1 n)))))


you should see
ping 0
ping 1
... etc
as expected.


then eval (while ping is running),
(+ 1 1) and you should see 2. 

but now the pings stop!
I have to type something and hit return before pings resume ... why is that?