bibliography citing code modification for automatisation

Question

In one of the answers here: How to make only cited bibitems appear?

user @UlrikeFischer gives a short solution to provide automation when not using bibtex, I copy her code here and it works for citing individual items. However it does not work for me when citing multiple items. I have added "\cite{a,b}" to her code snippet here, and this does not work for me, i.e. there is a question mark where I put b and reference b is not listed

Can Ulrike or anyone else help me out by adapting the code snippet so that also \cite{a,b} e.g. works? I don't really understand the latex code well enough to try and adapt it myself

Thanks a lot for your help, Hercule Poirot

ps: I know that I should use Bibtex or another automated package in future, so telling me that will not be helpful.

\documentclass{article}
\usepackage{xpatch}
\makeatletter
\xpretocmd\@citex{\csgdef{used@cite@#2}{x}}{}{\failed}
\newcommand\checkbibentry[2]{\ifcsdef{used@cite@#1}{\bibitem{#1}{#2}}{}}
\makeatletter
\begin{document}
\cite{a} \cite{c} \cite{a,b}
\begin{thebibliography}{99}
\checkbibentry{a}{some text to a}
\checkbibentry{b}{some text to b}
\checkbibentry{c}{some text to c}
\end{thebibliography}
\end{document}

Answer 1

You can map through the key list:

\documentclass{article}
\usepackage{xpatch}
\ExplSyntaxOn\makeatletter
\cs_new_protected:Npn\__poirot_mark_cite_as_used:n #1
 {
   \clist_map_inline:nn {#1}
    {
      \tl_gclear_new:c{g__poirot_cite_used_##1_tl}
    }
 }
\newcommand\checkbibentry[2]
 {
  \tl_if_exist:cT {g__poirot_cite_used_#1_tl}
   { \bibitem{#1}{#2} }
 }
\ExplSyntaxOff
\xpretocmd@citex{\csname __poirot_mark_cite_as_used:n\endcsname{#2}}{}{\failed}
\makeatother
\begin{document}
\cite{a} \cite{c} \cite{a,b}
\begin{thebibliography}{99}
\checkbibentry{a}{some text to a}
\checkbibentry{b}{some text to b}
\checkbibentry{c}{some text to c}
\end{thebibliography}
\end{document}