> is that correct, or should the production list be something like:

Yup, you are right.  I will change the grammar.

> Whether these cases are valid or not (personally, I think they should 
> be), we should add some tests for them.  '<' *is* currently valid, I 
> believe.

I agree, they should be valid.  I will add more test cases.

> The possibility of mixing native size/alignment with standard 
> size/alignment in a single format string makes me a bit uneasy

I agree.  It is hard for me to see how this might be used.  In any case,
the relevant part of the PEP that I was following is:

"Endian-specification ('!', '@','=','>','<', '^') is also allowed inside the string so that it can change if needed. The previously-specified endian string is in force until changed. The default endian is '@' which means native data-types and alignment. If un-aligned, native data-types are requested, then the endian specification is '^'."

However, I am not quite sure how to interpret the last sentence.

> Should the switch to '>' within the embedded struct be regarded as 
> local to the struct?

No, there is no notion of scope here.  A given specifier is active until the next one is found.

> Ah, it should have been:
> 
> assert(soself->s_tree != NULL);

D'oh!  I missed that when I merge over to py3k -- I started this work on trunk.  Thanks.