Hello Ulrike,

> 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. (I personally feel
\engine_if_... would be clearer for these, but the rest of the team are
as-yet unconvinced.)

> It seems to work fine. But I'm a bit unsure about the naming of the
> command. Is the prefix \ufcore OK?

You can pick any prefix you like at present. We do need some way of
logging these: I may make some proposals in the near-ish future.
-- 
Joseph Wright