如何在 Beamer 中将 `\mathrm` 与 `luatexja-preset` 结合使用？

Question

lualatex t.tex

对于以下代码

% t.tex
\documentclass{beamer}

\usepackage{luatexja-preset}

\begin{document}

\begin{frame}
$\mathrm{a}$
\end{frame}

\end{document}

导致以下错误

! LaTeX Error: Math alphabet identifier \mathrm is undefined in math version `n
ormal'.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.10 \end{frame}

t.tex如果我注释掉该行，编译时不会出现错误

\usepackage{luatexja-preset}

Answer 1

% t.tex
\RequirePackage{luatex85}
\documentclass[professionalfonts]{beamer}

\usepackage{luatexja-preset}

\DeclareMathAlphabet\mathrM{\encodingdefault}{\familydefault}{m}{n}
\let\mathrm\mathrM % not sure why I can't declare this directly
\begin{document}

\begin{frame}
a $a,\mathrm{a},\mathgt{a}$
\end{frame}

\end{document}