cerr or cout ?

right now the progress bar is outputed on cerr, the rational behind is that i don't want to pollute the printed text of my script

for example:
 `lua generatetext.lua > generatedtext.txt`
the bar is not present in generatedtext.txt + i got to see the progression on my terminal

but we could say the same for a script generating a lot of log... `lua server.lua 2> error.log`