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

I would find similar \xetex_or_luatex_if_engine:... quite useful but
as it doesn't exist I defined my local one:

\documentclass{scrartcl}
\usepackage{expl3}
\begin{document}

\ExplSyntaxOn

  
\prg_new_conditional:Nnn  \ufcore_xetex_or_luatex_if_engine: {T, F,
TF}
 {\bool_if:nTF
  {\c_luatex_is_engine_bool || \c_xetex_is_engine_bool}
  {\prg_return_true:}
  {\prg_return_false:}
 }

\ufcore_xetex_or_luatex_if_engine:TF{true}{false}
\ExplSyntaxOff

\end{document}

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


-- 
Ulrike Fischer