All is good and sorry if my reply seemed rude. It's just that this discussion pops up every few weeks... The main consensus seems to be that the writer composes the main melody and the lyrics. That's the song. Everything else (a cool guitar lick here or a string quartet playing a contra melody) is arrangement. In rock music it's unusual to give arranger credits, although some do. I also love all those little details. I mean, that's what we are here for, right?