[NTG-context] large delimiters for equation

Hello, I'd like to have delimiters that match the height of the fraction in an expression like this: \startformula c\Delta T = t\big{(} \frac{c}{v_s} - \frac{c}{v_f} \big{)}. \stopformula I looked in the docs and tried a few likely solutions without luck. Please accept my apologies if this is documented eleswhere. Thanks, Roger