[OS X TeX] Strange error

[Morten Høgholm]

On Mon, 12 Nov 2007 23:41:30 +0100, Ross Moore wrote:

Hello Ross,

> What I don't understand is the subtle difference that using
>   \begingroup ... \endgroup  makes in this context.
> How is it different to  { ... } ?   or  \bgroup ... \egroup  ?
>
> Most curious.

And rightly so! :-)  For some reason, DEK decided to implement {...} to  
act as a shorthand for \mathord{...} in math mode. And so nice TeX  
programmers like Didier Verna get cought by it sometimes when all they  
want is grouping but they wind up with more than they wished for. \bgroup  
and \egroup act as { and } in this case and produce an Ord.

Somewhat related are the well-known (for those who know too much!)
   \iffalse {\fi \iffalse`}=0 \fi
   \iffalse`{=0 }\fi
when it comes to opening special groups in alignments without accidentally  
introducing extra math atoms.

Cheers,
-- 
Morten