\ExplSyntaxOn
\sys_gset_rand_seed:n{42}
\ExplSyntaxOff
\DocumentMetadata{
 lang=en,
 testphase={phase-III,math},
 pdfversion=2.0,
 pdfstandard=ua-2,
 pdfstandard=a-4f,
 uncompress
}
\input{regression-test}
\documentclass{article}

\usepackage{unicode-math}

% suppress writing of luamml-mathml
\tagpdfsetup{math/mathml/luamml/write=false} %

% suppress mathml-AF reading
\tagpdfsetup{math/mathml/sources=} %
\tagpdfsetup{math/mathml/AF=false,attach-css=false}
\begin{document}
\ExplSyntaxOn

\luamml_structelem:
$a = b \quad
 \tagstructbegin{tag=mtext,stash}\tagmcbegin{}
 \luamml_annotate:en{nucleus=true,structnum=\tag_get:n{struct_num}}
  {\mbox{some~text~with~\emph{structure}}}
 \tagmcend\tagstructend 
$
\ExplSyntaxOff
\end{document}