来源：https://en.wikibooks.org/wiki/LaTeX/Macros
方括号中的参数其实就是默认参数。
使用方法：
\newcommand{name}[num][default]{definition}例子：
\newcommand{\wbalTwo}[2][Wikimedia]{
This is the Wikibook about LaTeX
supported by {#1} and {#2}!}
% in the document body:
\begin{itemize}
\item \wbalTwo{John Doe}
\item \wbalTwo[lots of users]{John Doe}
\end{itemize}输出：
This is the Wikibook about LaTeX supported by Wikimedia and John Doe!
This is the Wikibook about LaTeX supported by lots of users and John Doe!