### Optional arguments like `\section`

Optional arguments, in macros defined using `\newcommand`, don't quite work like the optional argument to `\section`. The default value of `\section`'s optional argument is the value of the mandatory argument, but `\newcommand` requires that you 'know' the value of the default beforehand.

The requisite trick is to use a macro in the optional argument:

    \newcommand\thing[2][\DefaultOpt]{\def\DefaultOpt{#2} ...}