`elsarticle` `\MSC` takes optional argument

[nschloe]

MWE:

\documentclass{elsarticle}

\begin{document}

\begin{frontmatter}
\begin{keyword}
  Lorem

  \MSC[abc] ipsum
\end{keyword}
\end{frontmatter}

\end{document}

LaTeX output:

LaTeXML output:

  <classification scheme="keywords">
Lorem
</classification>
  <classification scheme="MSC">[abc] ipsum
</classification>

(Note the literal treatment of [abc].)