Arithmetic track mix operations like: X = Y (+-*/) Z

“X = Y + Z” is just a normal mix-down.
“X = Y - Z” is mix-down after inverting one track (to make it -ve).
“X = Y / Z” will inevitably involve division by zero
“X = Y * Z” can be done using this Nyquist code on a stereo track …

(mult (aref *track* 1) 0.5 (sum 1 (aref *track* 0)))

There is an expression generator plugin, but it doesn’t modify existing audio,
it generates a waveform from a mathematical expression (formula).