cool

minor question:
- in the given patch __add__ uses __iadd__, but __sub__ doesn't use __isub__, which seems a bit weird.

maybe off-topic, but maybe not, because of _keep_positive(self):
- is there place for a non multi-set centric "Stats" object which is like Counter but with + and - actually behaving without the (in my use cases of Counter often counter intuitive) "> 0" stuff? (pun intended ;) ) Counter feels like a sub-class of Stats with the added _keep_positive(self).