I have wondered how to undefine existing commands in $\LaTeX$ for so long. Finally, I googled it up. It's easy. Simply ```latex \makeatletter \let\command\@undefined \makeatother ``` and the `\command` has been undefined. This does not cause an error when `\command` was undefined before. After that, you can merrily ```latex \newcommand{\command}{Hell Yeah.} ``` and be on your way.