Lars Hellström wrote:
> 2. I believe \xparse_prepare_next:w should be listed as a variable (or
> maybe "variable function"), since it is getting redefined rather
> frequently and seems to be keeping track of the state of the argspec
> parser automaton.

Thinking about this again, I realise that this can be done more clearly
without using \xparse_prepare_next:w at all. I've recoded that part of
xparse, also fixing a couple of bugs I spotted whilst doing it. I've
tried to improve the comments as well on the parts I've re-worked.
-- 
Joseph Wright