PATCH: Augmented assignment

[Huaiyu Zhu]

>But Guido suggested __add_ad__ in his latest posting on this subject, and it
>is ultimately his choice. If he comes up with a better name, or changes his
>mind in favor of whatever suggestion whoever made, it's a simple case of
>search-and-replace to fix it, anyway ;)

Before he makes final decision I'd make another suggestion

__add_by__

Same length.  A little less cryptical.  I've tried a few stituations and it
makes some sense.  Another possibility is __incr__ but this can't be done
systematically for all such operators.

Huaiyu