### 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} ...}

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