[Dev-luatex] start_run not working

I tried the following code: \directlua0{ local function simpleone() return "I'm number 1" end local function simpletwo() texio.write_nl("I'm number 1") end callback.register("start_run",simpletwo) } and got no result neither by replacing simpletwo by simpleone in the callback. Is it normal? The simpletwo function works fine with the callbacks "stop_run". -- Elie