我准备在 ACM 模板中定义一个命题，但是出现了这个错误，
未定义控制序列。\end{proposition}，
\documentclass[sigconf]{acmart}
\let\sigproof\proof\let\proof\relax
\let\sigendproof\endproof\let\endproof\relax
%
% defining the \BibTeX command - from Oren Patashnik's original BibTeX documentation.
\def\BibTeX{{\rm B\kern-.05em{\sc i\kern-.025em b}\kern-.08emT\kern-.1667em\lower.7ex\hbox{E}\kern-.125emX}}
% Rights management information.
% This information is sent to you when you complete the rights form.
% These commands have SAMPLE values in them; it is your responsibility as an author to replace
% the commands and values with those provided to you when you complete the rights form.
%
% These commands are for a PROCEEDINGS abstract or paper.
\copyrightyear{2019}
\acmYear{2019}
\setcopyright{acmlicensed}
\acmConference[Woodstock '18]{Woodstock '18: ACM Symposium on Neural Gaze Detection}{June 03--05, 2018}{Woodstock, NY}
\acmBooktitle{Woodstock '18: ACM Symposium on Neural Gaze Detection, June 03--05, 2018, Woodstock, NY}
\acmPrice{15.00}
\acmDOI{10.1145/1122445.1122456}
\acmISBN{978-1-4503-9999-9/18/06}
\usepackage{amsmath, amsfonts, amssymb, color, enumerate, amsthm, graphicx}
\usepackage{tikz, subcaption}
\usepackage{color, xcolor, soul, bm}
\PassOptionsToPackage{hyphens}{url}\usepackage{hyperref}
\hypersetup{
colorlinks = true, %Colours links instead of ugly boxes
urlcolor = blue, %Colour for external hyperlinks
linkcolor = blue, %Colour of internal links
citecolor = green %Colour of citations
}
\usepackage[normalem]{ulem}
\let\proof\sigproof
\let\endproof\sigendproof
\newtheoremstyle{sig}
{}
{}
{\itshape}
{}
{\scshape}
{.}
{.5em}
{#1 #2\thmnote{\quad(#3)}}
\theoremstyle{sig}
\newtheorem{problem}{Problem}
\newtheorem{theorem}{Theorem}
\newtheorem{corollary}{Corollary}
\newtheorem{assumption}{Assumption}
\newtheorem{proposition}{Proposition}
\newtheorem{lemma}{Lemma}
\newtheorem{remark}{Remark}
\newtheorem{definition}{Definition}
\newtheorem{example}{Example}
\usepackage{epsfig}
\usepackage{psfrag}
\DeclareMathOperator{\diag}{diag}
\usepackage{tcolorbox}
\newcommand{\JW}[1]{{\color{red}#1}}
\include{user_newcommands}
\usepackage{algorithm}
\usepackage{algpseudocode}
\allowdisplaybreaks
%\newcommand{\ShS}[1]{$\clubsuit$\footnote{SHREYAS: #1}}
%\newcommand{\mathcolorbox}[2]{\colorbox{#1}{$\displaystyle #2$}}
\def\endtheorem{\hspace*{\fill}~\IEEEQEDopen\par\endtrivlist\unskip}
\def\endlemma{\hspace*{\fill}~\IEEEQEDopen\par\endtrivlist\unskip}
\def\endproposition{\hspace*{\fill}~\IEEEQEDopen\par\endtrivlist\unskip}
\def\endexample{\hspace*{\fill}~\IEEEQEDopen\par\endtrivlist\unskip}
\def\endremark{\hspace*{\fill}~\IEEEQEDopen\par\endtrivlist\unskip}
\def\enddefinition{\hspace*{\fill}~\IEEEQEDopen\par\endtrivlist\unskip}
%
% These commands are for a JOURNAL article.
%\setcopyright{acmcopyright}
%\acmJournal{TOG}
%\acmYear{2018}\acmVolume{37}\acmNumber{4}\acmArticle{111}\acmMonth{8}
%\acmDOI{10.1145/1122445.1122456}
%
% Submission ID.
% Use this when submitting an article to a sponsored event. You'll receive a unique submission ID from the organizers
% of the event, and this ID should be used as the parameter to this command.
%\acmSubmissionID{123-A56-BU3}
%
% The majority of ACM publications use numbered citations and references. If you are preparing content for an event
% sponsored by ACM SIGGRAPH, you must use the "author year" style of citations and references. Uncommenting
% the next command will enable that style.
%\citestyle{acmauthoryear}
%
% end of the preamble, start of the body of the document source.
\begin{document}
\begin{proposition}\label{prop: L2-gain}
My proposition ...
\end{proposition}
\begin{sigproof}
My proof ...
\end{sigendproof}
\end{document}