extra backslashes in format dumps #2747
Description
The following error comes up if you try to InputDefinitions('siunitx', type=>'sty', noltxml => 1);.
\documentclass{article}
\begin{document}
\ExplSyntaxOn
\cs_new:Npn \tpmm_good_loop:N #1 {
Good~
\quark_if_recursion_tail_stop:N #1
\tpmm_good_loop:N }
\tpmm_good_loop:N \q_recursion_tail \q_recursion_stop
\par
\cs_new:Npn \tpmm_bad_loop:N #1 {
Bad?~
\peek_catcode:NTF \c_group_begin_token { failed } {
Bad~
\quark_if_recursion_tail_stop:N #1
\tpmm_bad_loop:N } }
\tpmm_bad_loop:N \q_recursion_tail \q_recursion_stop
\ExplSyntaxOff
%%1234
\end{document}To parse a key value, siunitx uses something like the bad loop, and the above in LaTeX becomes
Good
Bad? Bad
With LaTeXML, the recursion stops in the good loop, but something causes the peek (aka \futurelet) to disturb the quark in #1, so that the recursion no longer stops. We get the errors
Error:expected:Until:\\q_recursion_stop Missing argument Until:\\q_recursion_stop for Core::Definition::Expandable[\use_none_delimit_by_q_recursion_stop:w Until:\\q_recursion_stop] at (end of file)
Error:expected:\end{document} Input ended while environment document was open at String; line 1 col 0
and the output
<para xml:id="p1">
<p>Good</p>
</para>
<para xml:id="p2">
<p>Bad? Bad Bad? Bad Bad? Bad Bad? faileddocument</p>
</para>where the number of "Bad? Bad" is however many tokens there were before the end of the file.
I've been trying (and failing) to get the above to be free of expl3. But I also noticed the \\q_recursion_stop error has an extra backslash compared to the \use_none... or \end{document}. Indeed, my latex_dump.pool.ltxml has
I(E(C('\\use_none_delimit_by_q_recursion_stop:w'),Ps(P('Until','Until:\\\\q_recursion_stop',extra=>[T(C('\\q_recursion_stop'))])),T(),isLong=>1));whereas in LaTeX \show\use_none_delimit_by_q_recursion_stop:w gives
> \use_none_delimit_by_q_recursion_stop:w=\long macro:
#1\q_recursion_stop ->.In plain_dump, there are a few other suspicious cases of four backslashes, and in latex_dump there are many (eg Lt('\\\\allowbreak','\\relax');. Are these backslashes correct?
If they are correct, then I'm not quite sure where the error is coming from. And regardless, I'm not sure why peeking at the quark changes the stopping behavior.