Promoting xparse


\documentclass{article}
\usepackage{xparse}
\NewDocumentCommand\en{g}{%
\IfNoValueTF{#1}{\epsilon}{\epsilon_{#1}}%
}
\begin{document}
$$\en$$ and $$\en{stuff}$$.
\end{document}


This is clear, and does not need any knowledge of the internals of the LaTeX3 approach. So I’m going to keep promoting xparse for day to day LaTeX users, even if they aren’t using much code. Hopefully this will make life easier for them, and for me, and will be a real benefit now from the LaTeX3 work.