部分框架注释

Question

创建如下注释的最佳方法是什么：

梅威瑟：

\documentclass[]{article}
%\usepackage{amsmath}
\begin{document}
\newcommand\Start[1]{\ensuremath{\begin{array}{| l}
\hline #1 \end{array}}}

\newcommand\End[1]{\ensuremath{\begin{array}{l |}
#1 \\ \hline  \end{array}}}

\def\ra{\rightarrow}

\section{Looks not so good}
$\begin{array}{ *{9}{l} }
\Start{R1} & \ra & R2  & \ra & R3 & \ra & R4 &      &            \\
               &       & L1  & \ra & L2  & \ra & L3 & \ra & \End{L4} \\
%1 & 2 & 3 & 4 & 5 & 6 & 7 & 8  & 9 \\
\end{array}$

\bigskip
{\Huge $R1$}
\end{document}