You owe me one!
Remember all those beers your friends promised you when you helped them out? Remember how most of them never came? Beer is de facto the friends' money, and a debt paid is a friend kept.
So stop losing friends because of unpaid debts! Keep track of all those beers!
About the source
This is old code, put here just in case somebody would like to have a look at it. It is poorly documented and of low quality. It was put together on a sunday morning after a heavy saturday night out (the whole idea smells of Porpora).