[NTG-context] underbrace under matrix

[Jethro Djan]

Hello,

I want to get underbraces under matrices in a math alignment environment. I’ve reproduced my problem here:

 \definemathmatrix                                                                 
   [pmatrix]                                                                       
   [left={\left(\,},right={\,\right)}]           
                                  
 \starttext                                                                                                                                                           
 \placeformula[-]\startformula\startmathalignment                                  
   \NC\                                                                            
   A \NC=\underbrace{                                                              
   \startpmatrix                                                                   
     \NC 5\NC 5\NR                                                                 
     \NC -1\NC 7\NR                                                                
   \stoppmatrix}\NR                                                                
   \NC\NC=\underbrace{                                                             
   \startpmatrix                                                                   
     \NC 2\NC 1\NR                                                                 
     \NC 4\NC 2\NR                                                                 
  \stoppmatrix}\NR                                                                
 \stopmathalignment\stopformula                                                                                                                                     
  \stoptext      

How can I achieve this?

Jethro