I am trying to implement the lemma numbering solution in this question. I am currently compiling the document from a parent file such that:
\usepackage{chngcntr}
\usepackage{apptools}
\AtAppendix{\counterwithin{lemma}{section}}
\newtheorem{lemma}{Lemma}
\begin{appendices}
\include{Appendix1/appendix1}
\end{appendices}
where the appendix file has
\begin{lemma}
A lemma in appendix.
\end{lemma}
The lemma number remains sequential and doesn't change to reflect that it is in the appendix file...
