support rollback prior to 2019/10/01

[mrpiggi]

Following example fails with current release as it relies on the robust definition of \end (precisely \csname end \endcsname) which was introduced with 2019/10/01

\begin{filecontents*}{\jobname-sub.tex}
\documentclass[\jobname]{subfiles}
\begin{document}
\jobname-sub: sub text
\end{document}
\end{filecontents*}

\RequirePackage[2019/10/00]{latexrelease}
\documentclass{minimal}
\usepackage{subfiles}
\begin{document}
text

\subfile{\jobname-sub}

even more text
\end{document}

This PR resolves this issue.

[gsalzer]

Thanks for the patch, looks elegant. I will check it. On first sight, it seems to address what I had on my agenda, to resolve the open issues. So you have probably saved me some work.
I'm not sure how faithful the rollback mechanism with the package latexrelease really is. I would have expected that with older versions of LaTeX, there is also a problem with \ExplSyntaxOn/Off.

[mrpiggi]

Thanks for the patch, looks elegant. I will check it. On first sight, it seems to address what I had on my agenda, to resolve the open issues. So you have probably saved me some work.

You're welcome.

I'm not sure how faithful the rollback mechanism with the package latexrelease really is. I would have expected that with older versions of LaTeX, there is also a problem with \ExplSyntaxOn/Off.

AFAIK \ExplSyntaxOn comes with the format and is not affected at all. Actually, latexrelease “only” redefines some macro definitions to the state of the kernel at the given date.