[XeTeX] ifxetex package

[Will Robertson]

Ross Moore wrote: (regarding engines with eTeX extensions)

> Umm, how about the following engines:
>
>     tex4ht ,  lacheck ,  LaTeX2HTML ,  Omega , Lambda , Aleph
>     lamed , hyperlatex , t4ht
> (not sure about the 1st and last.)
>     and  texi2dvi ,  texi2html ,  etc.

You've convinced me that the ifxetex package, however minimal, is a  
good idea to act as a flag for programs like the above.

In many cases, the ifxetex package source won't be processed directly  
right? Oh, wait, Omega & Lambda don't have eTeX extensions either --  
but again, I don't think they're being used.

But I suppose for ultimate compatibility reasons, it would be a good  
idea to switch from
   \ifundefined\XeTeXversion...
to
   \expandafter\ifx\csname XeTeXversion\endcsname\relax\else...

In this case I think I've been being overly stubborn :)

Will

P.S. In regard to Bruno's comment on people using pre-0.5 versions of  
XeTeX -- as far as I'm concerned, that's simply impossible :)