也可以用其他名字替换旧命令，再重新定义，例如：
```
\let\oldA\A
\renewcommand{\A}[1]{\textbf{\oldA{#1}}}
```
参考 [Can I redefine a command to contain itself?](https://tex.stackexchange.com/questions/47351/can-i-redefine-a-command-to-contain-itself "Can I redefine a command to contain itself?")，使用 `letltxmacro` 宏包可以规避一些潜在的问题：
```
\usepackage{letltxmacro}
\LetLtxMacro{\oldemph}{\emph}
\renewcommand{\emph}[1]{\textbf{\oldemph{#1}}}
```
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