I found a Code in the internet that makes an environment and I can't solve the error. Please help me.
% arara: xelatex \documentclass[twoside]{book} \usepackage{TikZ} \usepackage{xcolor,mdframed} %\usepackage{xepersian} %\settextfont{XB Niloofar} \begin{document} \chapter{ftjfyg} \section{sdg} \mdfsetup{skipabove=\topskip,skipbelow=\topskip} \newrobustcmd\ExampleText{% An \textit{inhomogeneous linear} differential equation has the form %\begin{align} %L[v ] = f, %\end{align} where $L$ is a linear differential operator, $v$ is the dependent variable, and $f$ is a given non−zero function of the independent variables alone. } \newcounter{theo}[section] \newenvironment{theo}[1][]{% \stepcounter{theo}% \ifstrempty{#1}% {\mdfsetup{% frametitle={% \tikz[baseline=(current bounding box.east),outer sep=0pt] \node[anchor=east,rectangle,fill=blue!20] {\strut Theorem~\thetheo};}} }% {\mdfsetup{% frametitle={% \tikz[baseline=(current bounding box.east),outer sep=0pt] \node[anchor=east,rectangle,fill=blue!20] {\strut Theorem~\thetheo:~#1};}}% }% \mdfsetup{innertopmargin=10pt,linecolor=blue!20,% linewidth=2pt,topline=true, frametitleaboveskip=\dimexpr−\ht\strutbox\relax,} \begin{mdframed}[]\relax% }{\end{mdframed}} \begin{theo}[Inhomogeneous Linear] \ExampleText \end{theo} \begin{theo} \ExampleText \end{theo} \end{document} 
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