Am Thu, 4 Aug 2011 17:46:40 +0100 schrieb Joseph Wright:

>> l3basics contains \luatex_if_engine:... and \xetex_if_engine:... 

>> I would find similar \xetex_or_luatex_if_engine:
> I think you are after '\pdftex_if_engine:', as this deals with the case
> where the engine is pdfTeX or otherwise.

You mean \pdftex_if_engine will give true for all engines with the
exception of xetex + luatex? Even with etex or omega? In this case
it is what I'm after. But why does nobody use it? E.g. siunitx
\xetex_if_engine:F {

Ulrike Fischer