Re: [NTG-context] Bug when mathmatrix is used inside mathalign

27 Mar 2019

      On Wed, 27 Mar 2019, Hans Hagen wrote:
...
On 3/27/2019 4:56 PM, Aditya Mahajan wrote:
...
Hi,
It was reported on TeX.SE[1] that there is a bug when math matrix is used
inside a math align:
\starttext
\startformula
     \startalign[n=2,align={right,left}]
         \NC x
         \NC = a ~ long ~ sequence ~ of ~ terms
         \NR
         \NC y
         \NC = ok ~ left
         \NR
         \NC \startmatrix[left={\left\lbrack},right={\right\rbrack}]
                 \NC x \NR
                 \NC y \NR
             \stopmatrix
         \NC = why ~ right?
         \NR
         \NC y
         \NC = ok ~ left ~ again
         \NR
     \stopalign
\stopformula
\stoptext
gives
https://i.stack.imgur.com/ZTAp3.png
It appears that \startmatrix globally resets \NC. I haven't looked into
the code yet to see why this is happening.
you can play with
\unprotect
\newcount\c_math_eqalign_column_saved
\newcount\c_math_eqalign_first_saved
\unexpanded\def\math_matrix_start#1%
 {\begingroup
\c_math_eqalign_column_saved\c_math_eqalign_column
\c_math_eqalign_first_saved \c_math_eqalign_first
  \edef\currentmathmatrix{#1}%
  \dosingleempty\math_matrix_start_indeed}
\def\math_matrix_stop
 {\math_matrix_stop_processing
\global\c_math_eqalign_column\c_math_eqalign_column_saved
\global\c_math_eqalign_first\c_math_eqalign_first_saved
  \endgroup}
\protect
Yes, this works. But we still need \globalpushmacro\c_math_matrix_first, 
otherwise nested matrices have wrong distance.

https://mailman.ntg.nl/pipermail/ntg-context/2017/089631.html

So, the following appears to work correctly:

\unprotect
\newcount\c_math_eqalign_column_saved
\newcount\c_math_eqalign_first_saved

\unexpanded\def\math_matrix_start#1%
   {\begingroup
    \globalpushmacro\c_math_matrix_first
    \c_math_eqalign_column_saved\c_math_eqalign_column
    \c_math_eqalign_first_saved \c_math_eqalign_first
    \edef\currentmathmatrix{#1}%
    \dosingleempty\math_matrix_start_indeed}

\def\math_matrix_stop
   {\math_matrix_stop_processing
    \global\c_math_eqalign_column\c_math_eqalign_column_saved
    \global\c_math_eqalign_first\c_math_eqalign_first_saved
    \globalpopmacro\c_math_matrix_first
    \endgroup}
\protect

I am not sure why the assignments in math_matrix_start are not global but 
those in math_matrix_stop are global.

Aditya