Setting variables using arithmetic operators

Dear all,

I'm new in PsyToolkit and have a basic question:
Is it true that it is not possible to use arithmetic operators when setting variables? For example, this code is not possible:

options

  set &x 3

  set &y 4



task sometask
  set $z &x+&y


Is it somehow possible to bypass it?

Thank you very much,

Maliković Marko