stem_listen.lua

local stem = require("stem")
local event = require("event")
local modem = require("component").modem
local messages = {}
local server = stem.connect('stem.fomalhaut.me')
server:subscribe('stem-test')

modem.open(666)

event.listen('stem_message', function(_, channel_id, message)
  messages[message] = message
end)

while true do
  local _, _, _, _, _, msg = event.pull("modem_message")
  if messages[msg] then
    table.remove(messages,msg)
    print("OK!")
  else
    print("ERROR: "..msg)
  end
end