declare AddPortObject Call
{NewPortObjects AddPortObject Call}

InfoMsg={NewProgWindow "See ping-pong"}

fun {PingPongProc Other}
   proc {$ Msg}
      case Msg
      of ping(N) then
         {InfoMsg "ping("#N#")"}
         {Call Other pong(N+1)}
      [] pong(N) then
         {InfoMsg "pong("#N#")"}
         {Call Other ping(N+1)}
      end
   end
end

{AddPortObject pingobj {PingPongProc pongobj}}
{AddPortObject pongobj {PingPongProc pingobj}}
{Call pingobj ping(0)}