err_e_STACK should be a universal constant

[Jeroen Demeyer <J.Demeyer@UGent.be>]

Hello,

As far as I know, every GEN returned by the PARI library falls in one of 
these 3 cases:

* on PARI stack
* clone
* universal constant

The only exception is err_e_STACK which is effectively a universal 
constant but is_universal_constant() returns 0 for it. Is there a reason 
for that?

I am asking in the context of Python bindings for PARI. The existing 
cypari2 package used to always copy everything. Now I'm trying to 
improve the memory management by keeping objects on the PARI stack for a 
longer time. To do this, I need to distinguish between the 3 cases I 
mentioned above and err_e_STACK is the only object (that I found so far) 
where this fails.


Jeroen.