Thanks, Michael and Peter, for your explanations. As I said, I don't understand it perfectly. You've filled in the gaps in my knowledge and enlightened everyone else too. Thanks!

I'm assuming this is going into a FAQ somewhere, Tony?

Save the whales. Feed the hungry. Free the mallocs.
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Owner of Mark I empeg 00061, now better than ever - (Thanks, Rod!) - and Karma 3930000004550