The Plain TeX
$$ \sum_{i \in S \atop j \in T} i $$
seems to be usually typeset mathematically in LaTeX using the \substack macro from the AMS maths macros (accessed using \usepackage{amsmath})
\[ \sum_{\substack{i \in S\\ j \in T}} i \]
which gives slightly different results, most importantly in that the subscripts are scriptsized in the latter, but scriptscriptsized in the former.
Results like these can be obtained in ConTeXt using matrices, e.g.:
\def\scriptsize{\switchtobodyfont[7pt]}
\startformula
\sum_{\startmathmatrix
\NC \scriptsize i \in S \NR
\NC \scriptsize i \in T \NR
\stopmathmatrix}
i
\stopformula
But this is clunky (not to mention my code above giving ugly spacing) and defining a macro to do this risks being ad hoc. Is there an attractive and idiomatic way to do this in ConTeXt? I'd be most interested in a reasonably well-established way of translating AMS maths into ConTeXt.
