 where  φC  is the formula that defines the class C. It is important that in this definition x ranges over sets and not classes; the formal language used does not directly talk about classes in the first place. For example, ℘V = V,  and ℘(WF) = WF. We also define the action of other operations on classes in the same general way. For example, the finite power set ℘fin  takes a class C to the class of finite subsets of  C. 