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I don't think that "lazy-binding + leftmost iterable 
precomputation" makes sense. Just adding another for-loop 
to the example shows the reason.

>>> x = 10
>>> g = ((i,j) for i in range(x) for j in range(x))
>>> x = 5
>>> list(g)
Here, if j is iterated only to 5 when i is iterated to 10, I think 
it would make users confused.

I think "early-binding + leftmost iterable precomputation" 
makes sense, but "lazy-binding + leftmost iterable 
precomputation" does not make sense IMHO.

About renumbering testlist_gexp in graminit.h, it's not what I 
did but it's auto-generated by pgen(Parser/pgenmain.c). 
Although it makes another file (symbol.py) need to be 
changed (and thus makes cvs log a bit dirty), it's tool-
generated file, so I did it like that. If you think it's better to 
do it without re-numbering, let me know or you could do it 
yourself.  ;^)