Help variables in weak form

Hi,

I'm wondering how to insert a help variable in the weak form. I am solving for three variables u1, u2 and u3. However, my weak form integrand is a complicated function of these variables. So I prefer not to enter everything in the "weak expression" field at once.

It should surely be possible to introduce some intermediate variables such that the expression for the weak form integrand is simpler. Alas I have been unable to define these intermediate variables somewhere.

Does anyone know how to do this?

Thanks!