Petr, I realize that you are in a difficult position. You'll end up disappointing either me or Mark... I don't know if the steering council or somebody else has a good idea to deal with this situation.
Jeroen has time
Speaking of time, maybe I should clarify that I have time until the end of August: I am working for the OpenDreamKit grant, which allows me to work basically full-time on open source software development but that ends at the end of August.
Here again, I mostly want to know if the details are there for deeper reasons, or just points to polish.
I would say: mostly shallow details. The subclassing thing would be good to resolve, but I don't see any difference between PEP 580 and PEP 590 there. In PEP 580, I wrote a strategy for dealing with subclassing. I believe that it works and that exactly the same idea would work for PEP 590 too. Of course, I may be overlooking something...
I don't have good general experience with premature extensibility, so I'd not count this as a plus.
Fair enough. I also see it more as a "nice to have", not as a big plus.