This is pretty cool (need... (11 Aug 2002)
There are very few numbers which can be produced from the system of subtracting the digits of a number. 10-19 give an answer of 9, 20-29 give an answer of 18 and so on for multiples of 9. This reduces the possibility set drastically.
The next thing to realise is that the possibilities are spaced out evenly and all the possibilities have the same symbol. Quite neat.