[NTG-context] Aaligned numbers in a matrix?

[Bruce Horrocks]

I'm using the following MWE to produce a matrix:

\definemathmatrix[bmatrix]
  [matrix:brackets]
  [simplecommand=bmatrix]
\starttext
$ a \rightarrow
  \bmatrix{1, 2, 3, 4; 
           2, 1, 4, 3;
           3,-4, 1,-2;
           4,-3, 2,-1} $
\stoptext

The columns with negative numbers in are centred and I would rather have them right-aligned so that the digits stack up above each other and the negative signs stick out to the left, so to speak. Is there an easy way to achieve this?

—
Bruce Horrocks
Hampshire, UK