[NTG-context] Aaligned numbers in a matrix?

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