mdframed 和脚注

Question

我使用基于 mdframed 的示例环境，该环境使用希腊字符来表示此环境中的脚注。问题是，当我开始将 xelatex（UTF-8 编码）与 Minion Pro 字体一起使用时，脚注标记不会像往常一样打印为上标，而是打印为普通字母。请参阅附件 MWE。可以解决这种情况吗？非常感谢。

\documentclass[a4paper,twoside,11pt]{memoir}
\usepackage{xltxtra}
\usepackage[nonumeralsign]{xgreek}
\usepackage{ucharclasses}
\setTransitionsForGreek{\setlanguage{monogreek}}{}
\setTransitionsForLatin{\setlanguage{american}}{}
\setmainfont[Mapping=tex-text]{Minion Pro}
\usepackage{color}
\usepackage[usetwoside]{mdframed}
\usepackage{graphicx}
\usepackage{titletoc}
\usepackage{pstricks}
\newcommand{\mple}[1]{\blue{#1}\blackink}
\def\blackink{\special{color cmyk 0 0 0 1.}}
\def\whiteink{\special{color cmyk 0 0 0 0}}

\newcounter{example}[chapter]
\def\theexample{\mple{\thechapter-\arabic{example}}\blackink}
\newenvironment{example}[2][]{\refstepcounter{example}\par\medskip\noindent
\mdfsetup{frametitleaboveskip=-0.5\ht\strutbox,
frametitlealignment=\raggedright }
\begin{mdframed}[hidealllines=true,backgroundcolor=blue!8]
\cyan\hrulefill\blackink \\
\colorbox{cyan}{\textbf {\whiteink{\large{Example~\theexample}\rule[-6pt]{0ex}{4ex} }\blackink     #1}} \smallskip $\quad$ \textbf {\mple{#2}} \\[-3.5pt]
\cyan\hrulefill\blackink}{${}$\\\cyan\hrulefill\blackink\end{mdframed}}

\begin{document}

\begin{example}{Sample example.}

This is just an example\footnote{This is a footnote.} to show the problem.
\end{example}

\end{document}