我使用以下代码时遇到错误。编辑器 WinEdt8.0 给出错误消息，表示无法识别该命令\Longstack。我是否缺少某个包？
在出现此错误之前，它出现了另一个错误：
LaTeX error: File `listofitems.sty' not found
Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: sty)
Enter file name:
这是代码。
\documentclass{amsart}
\usepackage{adjustbox}
\usepackage{mathtools}
\usepackage{array}
\usepackage{makecell}
\usepackage{stackengine}\setstackEOL{\cr} %EOL is abbreviation for "end of line."
\begin{document}
\setlength\extrarowheight{2pt}
\stackon{%
\begin{tabular}{|| c !{\vrule width0.8pt} c | c | c||} \hline
Year & 1985 & 1986 & 1987 \\ \Xhline{0.8pt}
Store $A$ & 250 & 400 & 750 \\ \hline
Store $B$ & 500 & 1,000 & 1,250 \\ \hline
\end{tabular}
}{\bfseries\Longstack{Number of Premium Members}}
\end{document}