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Armin, I don't understand your use case.  Can you be more 
explicit?  Best I can guess, you're using Python longs on a 
32-bit box to store positive integers with bit 2**31 set.  But 
there's no change in memory burden for those (rounds up to 
24 bytes either way), so that can't be what you mean.  
Maybe you're using Python longs to store _all_ integers, no 
matter how small they are?  But in that case you weren't 
serious about memory use to begin with.

Michael, I got confused at the start of the patch.  When you 
changed the comment

SUM(for i=0 through abs(ob_size)-1) ob_digit[i] * 2**(SHIFT*i)

to

sign * SUM(for i=0 through ob_size) ob_digit[i] * 2**(SHIFT*i)

you dropped the "-1" as well as the "abs()".  That wasn't 
intended, was it?

Was also confused by why you dropped the "zero is 
represented by ob_size == 0." comment.

It would be very helpful to rename the new member, e.g., 
as "long_sign".  Then it's easy to find references in the code 
with an editor search.