Re: getting metafun positions within (displayed) math mode

[Aditya Mahajan <adityam@umich.edu>]

On Thu, 16 Nov 2017, Sanjoy Mahajan wrote:

> However, the \hpos hides its argument in an \hbox, and thus hides it
> from the math layout.  Thus, I have to do something like this minimal
> example:
>
> \starttext
> \startformula
>  \sum_0^1                      % what it should look like
>  \quad
>  \hpos{n1}{$\displaystyle\sum$}_{\hpos{n2}{$0$}}^{\hpos{n3}{$1$}}
> \stopformula
> \stoptext
>
> The hpos'ed version after the \quad looks different, even after the
> \displaystyle hack.
>
> Is there a version of \hpos for math mode?  That would be the cleanest
> solution, because who knows what else would be needed for other cases,
> even if the limits can be placed correctly.
>
> Or is there a better way to annotate equations?

No time for a detailed answer, but what you need is the following:

\starttext
\startformula
   \sum_0^1                      % what it should look like
   \quad
   \mathop{\hpos{n1}{\mathsurround\zeropoint$\displaystyle\sum$}}_{\hpos{n2}{\mathsurround\zeropoint$\scriptstyle0$}}^{\hpos{n3}{\mathsurround\zeropoint$\scriptstyle1$}}
\stopformula
\stoptext

So what you need to do is find the mathmode of the current symbol 
(mathbin, mathord, etc) and find the mathstyle of the current symbol 
(\displaystyle, \scriptstyle, \scriptscriptstyle, etc), and reapply those.

There is some code in the core to do detect whether the current symbol is 
a bin or not (and perhaps that can be adapted to see if the current symbol 
is an op or not). But a simpler solution will be use luatex to traverse 
the mathlist after it has been typeset (but I don't understand who that 
works).

Of course, an ideal solution will be to define a macro \mpos{...} that 
does this behind the scenes.

Aditya
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