rectangle equation

Hello,

I am aiming to create a rectangle equation that depends on x and y in COMSOL 4.3a. I read an article in which the author created a similar one shown below:

min(min(x,8-x),min(y,6-y))

in FEMLAB 3.0a. I tried the same equation in my COMSOL, but the solver did not seem to create a rectangle, nor gave an error related to the equation. Could you suggest a way of writing an equation for a rectangle?

Thanks,
Onur S. Aslan