Having .emacs distinguish betweeen LaTeX and XeLaTeX

[Haines Brown <haines@histomat.net>]

I would like to specify the TeX engine with the emacs command line, such
as with C-c C-c latex and C-c C-c xelatex, rather than having it only
call whichever TeX-engine is specified in AUCTeX configuration or in
.emacs init file. At present I can only switch between processing LaTeX
and XeLaTeX files by inserting or commenting a line in .emacs (setq
TeX-engine 'xetex) and reload emacs.

Simplest would be to have AUCTeX not specify any TeX-engine and instead
use the engine specified at the command prompt. There does not seem to
be any way to do this, and so an alternative might be for .emacs to
specify the TeX engine by distinguishing between LaTeX and XeLaTeX
files.

Someone using the Jed editor does this. He sets the default TeX-engine to
xetex if the document uses packages peculiar to XeTeX:

   if (re_fsearch("\\usepackage.*unicode-math") or
       re_fsearch("\\usepackage.*fontspec") or
       re_fsearch("\\usepackage.*polyglossia")
      )
      define_blocal_var("latex_output", "xetex");

Is it possible to something comparable to this if construction in .emacs
init? I know nothing of lisp, but guess it would be something like:

   ( if 
        (search-forward "\usepackage*fontspec") or
        (search-forward "\usepackage*polyglossia")
     (setq TeX-engine 'xetex))

Haines Brown