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Rats had featured largely in the history of Ankh-Morpork. Shortly before the Patrician came to power there was a terrible plague of rats. The city council countered it by offering twenty pence for every rat tail. This did, for a week or two, reduce the number of rats--and then people were suddenly queing up with tails, the city treasury was being drained, and no one seemed to be doing much work. And there still seemed to be a lot of rats around.

Lord Vetinari had listened carefully while the problem was explained, and had solved the thing with one memorable phrase which said a lot about him, about the folly of bounty offers, and about the natural instinct of Ankh-Morporkians in any situation involving money: "Tax the rat farms."

-- Soul Music, by Terry Pratchett