[NTG-context] "[" at beginning of formula ?

Hello, I'd like to typeset [\partial,B]=0 but \starttext \placeformula \startformula 0= [ A ,B ] = 0 \stopformula gives just "=0" (see below) while \stoptext \starttext \placeformula \startformula 0= [ A ,B ] \stopformula \stoptext works. What is the meaning of [ at the beginning of a formula, and how do I get rid of it? Matthias