Subclassing int for a revolving control number

[Chris Cioffi]

Hello all!

I'm trying to subclass int such that once it reaches a certain value,
it flips back to the starting count.  This is for use as an X12
control number.  Subclassing int and getting the display like I want
it was trivial, however what's the best way to make it such that:
990 + 1 = 991 
991 + 1 = 992
...
998 + 1 = 999
999 + 1 = 001

Should I set any of the special functions like __sub__ to raise
NotImpemented and just make my own custom assignment and __add__
functions?  (The other fucntions would make no sense in this
context...)

Is there an "on change" kind of meta-function?  (Like the onChange
event in a GUI text control)

Chris
-- 
Still searching for an even prime > 2!