[NTG-context] Math: Spacing around colons

Dear List, I need a binary math operator with narrower spacing then the default. My feeble attempts below all lead to the same spacing except for the last one which however is not symmetric. Thanks for any hints! Matthias \definemathcommand [col] [rel] {\mathstylevcenteredhbox\colon} \starttext \startformula (a \centercolon b \centercolon c) \stopformula \startformula (a \col b \col c) \stopformula \startformula (a : b : c) \stopformula \stoptext