If ABSTRACT has cite, EJP documentclass will conflict with natbib usepackage.

[ChoCho66]

As the title says, if there is \cite in \ABSTRACT, the compilation will not succeed.
Below is the wrong message.

LaTeX Error: \begin{abstract} on input line 11 ended by \end{document}.

 
‪./main.tex, 27‬
See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.27 \end{document}
                   
Your command was ignored.
Type  I <command> <return>  to replace it with another command,
or  <return>  to continue without it.

---

The following is a small template.

• main.tex

  \documentclass[EJP]{ejpecp} % replace ECP by EJP if needed.
  
  \input{other.tex}
  
  \usepackage{natbib}
  
  \ABSTRACT{abstract
  \cite{Revuz-Yor}
  abstract.}
  
  \begin{document}
  
  Test \cite{Revuz-Yor}.
  
  \citep[See][Theorem 1.6]{Revuz-Yor}.
  
  \begin{thebibliography}{99}
  
  \bibitem[Revuz and Yor, 1999]{Revuz-Yor}
  Revuz, D. and Yor, M. (1999). 
  \newblock {Continuous Martingales and Brownian Motion.}
  \newblock (3rd ed.) Springer-Verlag
  \newblock Berlin.
  
  \end{thebibliography}
  
  \end{document}

• other.tex

  \SHORTTITLE{Introduction to the \emph{ejpecp} Class}
  
  \TITLE{Introduction to the \emph{ejpecp} Class Version 1.11.1\support{Supported
      by the Institute of Mathematical Statistics (IMS) and the Bernoulli
      Society.}\
      \thanks{Current maintainer of class file is
        \href{https://vtex.lt}{VTeX, Lithuania}. Please send all queries to
        \href{mailto:latex-support@vtex.lt}{\texttt{latex-support@vtex.lt}}.}} % \thanks is optional. Insert line breaks with \\
  
  \AUTHORS{%
    Krzysztof~Burdzy\footnote{University of Washington, United States of America.
      \EMAIL{burdzy@math.washington.edu}}
    \and %% remove this line and below if single author
    Djalil~Chafa\"{\i}\footnote{Universit\'e Paris-Dauphine,
      France. \BEMAIL{djalil@chafai.net} \url{http://djalil.chafai.net}}}%AUTHORS
  
  \KEYWORDS{EJP ; ECP ; typesetting ; LaTeX} % Separate items with ;
  
  \AMSSUBJ{NA} % Edit. Separate items with ;
  %\AMSSUBJSECONDARY{FIXME:} % Optional, separate items with ;
  
  \SUBMITTED{January 2, 2013} % Edit.
  \ACCEPTED{December 13, 2014} % Edit.
  
  \VOLUME{0}
  \YEAR{2020}
  \PAPERNUM{0}
  \DOI{10.1214/YY-TN}

• ejpecp.cls is the file https://github.com/vtex-soft/texsupport.ims-ejpecp/blob/master/ejpecp.cls