On 04/08/2011 18:10, Ulrike Fischer wrote:
> You mean \pdftex_if_engine will give true for all engines with the
> exception of xetex + luatex? Even with etex or omega?

Only three engines are supported: pdfTeX, XeTeX and LuaTeX, and even
then only in sufficiently-new versions. The reason is ultimately that we
need \pdfstrcmp or equivalent functionality. (expl3 will bail out if it
can't find \pdfstrcmp, \strcmp or Lua to emulate it.)

> In this case
> it is what I'm after. But why does nobody use it? E.g. siunitx
> contains 
> \xetex_if_engine:F {
>   \luatex_if_engine:F....  

That code was written before \pdftex_if_engine: was added to expl3. I
should alter it, I guess.
Joseph Wright