This is actually a reply to fever.

The reference to $count is actually pure guesswork on my part. The problem is that I cannot find a way to say "if the input exists - take it out, if it doesn't - put it in". Obviously inclusion of the $count mechanism is more problematical than I'd anticipated but I can't find another approach which comes close to letting me say, "while values of @in match values of @db or the converse".

That, I think is the crux of the problem!