Two collaboration worlds display a moving ball, one of them should rest. 

Pass Through (Distributed) Version
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 Two screens pop up and a ball moves from the bottom to the top, on each of
 them. When one reaches the top, it rests and sends a signal to the other
 to 'go. This means only one of the worlds will have a moving ball, the
 other one rests. 

 use ../pass-through.ss 

 World and Messages: 
 ;; World   = Number | 'resting 
 ;; Message = 'go

Arbitrated Version
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 Two screen pop up. The server sends one of them a go signal and the other
 one a rest signal. Until then both move so I can use the same shared
 code. 

 use ball-universe.ss 

 World and Messages: 
 ;; World    = Number | 'resting 
 ;; ReceivedMessage = 'go
 ;; SendMessages    = ... any token will do ... 

 Server: 
 ;; ReceivedMessages = ... any token will do ... 
 ;; SendMessages     = 'go