This ended up being a lot less pretty than I wanted. Maybe I should just define the class. 

The keyed_args thing may be a bad idea, because you quite often want to use a different key.

Any thoughts on how this should be expressed? Frankly, what I would like to do is to just write `Q.zero(Q.zero**Q.positive)`. I'll have to think about how to do that without making too big of a mess.