On 16/07/2015 20:47, David Carlisle wrote: > A simplified example, should anyone be looking at this is: > > \listfiles > \documentclass{article} > \usepackage{xparse} > \DeclareDocumentCommand\foo{mO{xxx}}{[#1][#2]} > \usepackage{array} > \begin{document} > a $x$ > > \tracingmacros2 > > %\begin{tabular}{>{\foo{zz}[]}l}% works > \begin{tabular}{>{\foo{zz}}l}% fails > text > \end{tabular} > \end{document} I think I know where to look at this but not quite what is up! I guess as this is linked to xparse it's one for me: I'll log on GitHub and investigate. -- Joseph Wright