procedure main(A)
    fName := A[1]|"index.txt"
    p := thread produce(fName)
    c := thread consume(p)
    every wait(p | c)
end

procedure produce(fName)
    every !open(fName)@>>    # drop messages in p's outbox (blocking whenever box is full)
    @>>                      # Signal consumer that p is done
    write("count is ",<<@)   # block until message in p's inbox
end

procedure consume(p)
    i := 0
    while write(\<<@p) do (i+:=1)  # loop until empty message in p's outbox
                                   #    (blocking until each message arrives)
    i@>>p                          # put row count into p's inbox
end