[dev-context] "Improved" substack macro

[Aditya Mahajan]

Hi Hans and Taco,

The \startsubstack macro in core-mat places the subscript lower than 
the atop. See for example

\startformula
\sum_{\startsubstack a \NR b \NR \stopsubstack}
\sum_{\scriptstyle a \atop \scriptstyle}
\stopformula

I have tried to get the correct spacing by copying what amsmath does. 
When you have time, can you look if it makes sense to provide both the 
substack macros.

Aditya


\def\startsubstackA
   {\vcenter\bgroup
     \baselineskip\fontdimen10 \scriptfont\plustwo
     \advance\baselineskip\fontdimen12 \scriptfont\plustwo
     \lineskip\plusthree\fontdimen8 \scriptfont\plusthree
     \lineskiplimit\lineskip
     \let\stopmathmode\relax
     \def\NC{\domatrixNC}%
     \def\MC{\domatrixNC\startmathmode}%
     \global\let\domatrixNC\dodomatrixNC
     \def\NR
       {\stopmathmode
        \global\let\domatrixNC\dodomatrixNC
        \crcr}%
     \mathsurround\zeropoint
     \everycr\emptytoks
     \halign\bgroup\hfil$\scriptstyle##$\hfil\crcr}

\def\stopsubstackA
   {\crcr
    \egroup
    \egroup}

\starttext

\startformula
   \sum_{\startsubstack a \NR b \NR c \NR \stopsubstack}
   \sum_{a}
   \sum_{\startsubstackA a \NR b \NR c \NR \stopsubstackA}
   \sum_{\scriptstyle a \atop \scriptstyle b}
\stopformula

\stoptext