我正在使用《物理学会议系列期刊》的模板撰写一篇论文，可以通过此链接获取：
https://drive.google.com/drive/folders/17dfkKChHJOHbx1HT-uVMZsA41l2IthiW
每当我想使用证明环境时，都会出错。我的工作代码如下：
\documentclass[a4paper]{jpconf}
\usepackage{graphicx,amsthm}
\begin{document}
\title{Preparing a paper using \LaTeXe\ for publication in \jpcs}
\author{Jacky Mucklow}
\address{Production Editor, \jpcs, \iopp, Dirac House, Temple Back, Bristol BS1~6BE, UK}
\ead{[email protected]}
\begin{abstract}
All articles {\it must} contain an abstract. This document describes the preparation of a conference paper to be published in \jpcs\ using \LaTeXe\ and the \cls\ class file. The abstract text should be formatted using 10 point font and indented 25 mm from the left margin. Leave 10 mm space after the abstract before you begin the main text of your article. The text of your article should start on the same page as the abstract. The abstract follows the addresses and should give readers concise information about the content of the article and indicate the main results obtained and conclusions drawn. As the abstract is not part of the text it should be complete in itself; no table numbers, figure numbers, references or displayed mathematical expressions should be included. It should be suitable for direct inclusion in abstracting services and should not normally exceed 200 words. The abstract should generally be restricted to a single paragraph. Since contemporary information-retrieval systems rely heavily on the content of titles and abstracts to identify relevant articles in literature searches, great care should be taken in constructing both.
\end{abstract}
\section{Introduction}
These guidelines show how to prepare articles for publication in \jpcs\ using \LaTeX\ so they can be published quickly and accurately. Articles will be refereed by the \corg s but the accepted PDF will be published with no editing, proofreading or changes to layout. It is, therefore, the author's responsibility to ensure that the content and layout are correct. This document has been prepared using \cls\ so serves as a sample document. The class file and accompanying documentation are available from \verb"http://jpcs.iop.org".
\begin{proof}
Amirul
\end{proof}
\end{document}
产生Undefined control sequence. \end{proof}错误。日志文件给出以下错误：
! Undefined control sequence.
\qedsymbol ->\openbox
我怎么解决这个问题？