8
Jan
What do you expect if you run LaTeX on the following?
\documentclass{article}
\begin{document}
This is a $(\frac13, \frac23)$-separator.
\end{document}
Hint: it works.
P.S. In my experience, this “trick” is most useful for constant fractions with a one-digit numerator and a one-digit denominator. When in doubt, just use the braces, or you can learn why and how this works once and for all.
