# How to change the theorem counter, theorem name in a specific case

I have used GMS class to write design a math book. In that book, the author has a special theorem, as following:

\documentclass{gsm-l}
\newtheorem{theorem}{Theorem}
\newtheorem{lemma}[theorem]{Lemma}
\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}
\newtheorem{example}[theorem]{Example}
\newtheorem{xca}[theorem]{Exercise}
\theoremstyle{remark}
\newtheorem{remark}[theorem]{Remark}
\numberwithin{section}{chapter}
\numberwithin{equation}{chapter}
\setcounter{tocdepth}{2}
\makeindex
\begin{document}
\title{}
\maketitle

\section{Indent the subsection}
\begin{theorem}Here is theorem 1
\end{theorem}
\begin{theorem}This theorem must be numbered as theorem 2A
\end{theorem}
\printindex
\end{document}


As you can see in the MWE, there are two theorems. I want to number the second theorem to theorem 2A, and the next theorem to that is theorem 3 as usual.

Thanks.

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