deactivate underscores when expanding natbib \bibitem optional labels

MANIFEST

@@ -1674,6 +1674,9 @@ t/structure/lit.bib
 t/structure/crazybib.pdf
 t/structure/crazybib.tex
 t/structure/crazybib.xml
+t/structure/crazybibkeys.pdf
+t/structure/crazybibkeys.tex
+t/structure/crazybibkeys.xml
 t/structure/csquotes.pdf
 t/structure/csquotes.tex
 t/structure/csquotes.xml

lib/LaTeXML/Package/natbib.sty.ltxml

@@ -528,7 +528,7 @@ DefPrimitiveI('\reset@natbib@cites', undef, sub {
 
 # Kinda rediculous simulating this here (we can almost pull it off in TeX)
 # but the TeX tends to be quite brittle in the wilds of arXiv, so...
-DefMacro('\lx@NAT@parselabel Semiverbatim {}', sub {
+DefMacro('\lx@NAT@parselabel {}{}', sub {
     my ($gullet, $label, $key) = @_;
     my ($number, $year, $authors, $fullauthors);
     my $bare   = 1;
@@ -560,7 +560,10 @@ DefMacro('\lx@NAT@parselabel Semiverbatim {}', sub {
         ($a1, @tokens) = NAT_peel_arg(@tokens);
         $authors = $a1; $year = Tokens(@tokens); $bare = 0; } }
     if ($bare) {
-      @tokens = Expand($label)->unlist;
+      # Goal: deactivate underscores
+      #  (but not quite Semiverbatim, as math should be active things like \ddot{a})
+      my @safe_tks = map { Equals($_, T_SUB) ? T_OTHER("_") : $_ } $label->unlist;
+      @tokens = Expand(Tokens(@safe_tks))->unlist;
       # \bibitem[Jones et al.(1990)]{key}...
       # \bibitem[Jones et al.(1990)Jones, Baker, and Williams]{key}...
       # \bibitem[Jones et al., 1990]{key}...

t/structure/crazybibkeys.tex

@@ -0,0 +1,14 @@
+\documentclass{article}
+\usepackage{natbib}
+\begin{document}
+% These could cause problems if used with \cite, but are OK standalone in {thebibliography}.
+\begin{thebibliography}{}
+
+\bibitem[_xy(1899)]{_xyz_1899}
+ A name of something. (accessed Nov 01, 1899).
+
+\bibitem[Ex$\ddot{a}$mple(1899)]{mathy}
+  Another name.
+
+\end{thebibliography}
+\end{document}  

t/structure/crazybibkeys.xml

@@ -0,0 +1,51 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<?latexml class="article"?>
+<?latexml package="natbib"?>
+<?latexml RelaxNGSchema="LaTeXML"?>
+<document xmlns="http://dlmf.nist.gov/LaTeXML">
+  <resource src="LaTeXML.css" type="text/css"/>
+  <resource src="ltx-article.css" type="text/css"/>
+  <bibliography xml:id="bib">
+    <title>References</title>
+    <biblist>
+      <bibitem key="_xyz_1899" xml:id="bib.bib1">
+        <tags>
+          <tag role="number">1</tag>
+          <tag role="year">1899</tag>
+          <tag role="authors">˙xy</tag>
+          <tag role="refnum">˙xy (1899)</tag>
+          <tag role="key">_xyz_1899</tag>
+        </tags>
+        <bibblock>
+A name of something. (accessed Nov 01, 1899).
+</bibblock>
+      </bibitem>
+      <bibitem key="mathy" xml:id="bib.bib2">
+        <tags>
+          <tag role="number">2</tag>
+          <tag role="year">1899</tag>
+          <tag role="authors">Ex<Math mode="inline" tex="\ddot{a}" text="ddot@(a)" xml:id="bib.bib2.m1">
+              <XMath>
+                <XMApp>
+                  <XMTok name="ddot" role="OVERACCENT">¨</XMTok>
+                  <XMTok font="italic" role="UNKNOWN">a</XMTok>
+                </XMApp>
+              </XMath>
+            </Math>mple</tag>
+          <tag role="refnum">Ex<Math mode="inline" tex="\ddot{a}" text="ddot@(a)" xml:id="bib.bib2.m2">
+              <XMath>
+                <XMApp>
+                  <XMTok name="ddot" role="OVERACCENT">¨</XMTok>
+                  <XMTok font="italic" role="UNKNOWN">a</XMTok>
+                </XMApp>
+              </XMath>
+            </Math>mple (1899)</tag>
+          <tag role="key">mathy</tag>
+        </tags>
+        <bibblock>
+Another name.
+</bibblock>
+      </bibitem>
+    </biblist>
+  </bibliography>
+</document>