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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: e-TeX low-level expertise needed Re: Allocation of registers
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On 15/09/2010 21:15, Chris Rowley wrote:
> So what happens when e-TeX reads  \reserveinsert<bignum>  {<stuff>} ??

'Undefined control sequence'! The macro is called \reserveinserts :-) It 
just marks a set of count/dimen/skip/box spaces as not available for 
general use. So you can put any number in here without an error 
(provided it's less that \maxint, of course). It just doesn't work, in 
the sense you don't gain any extra \insert's.

I did a few tests. As the e-TeX manual says, you can't \insert beyond 
254. \insert255 gives the special 'You can't \insert255' error (as 
without e-TeX), and anything higher is a bad register code. (The 
{<stuff>} seems to vanish.)

So the important point for me is that \insert's are still limited with 
e-TeX, not as bad as \write's but much more so than \count's, etc. Thus 
you have to reserve a certain number of 'low' registers (count, dimen, 
skip, box) for use with inserts. The \reserveinserts macro does that 
with the e-TeX package, so for expl3 this is probably the way to go. For 
LaTeX3 as a format I guess the current allocation routine will need to 
be altered when we actually get to needing inserts. In both cases the 
question is then 'how many inserts is enough'? Morten suggests 30, which 
is probably sufficient at the moment.
--
Joseph Wright