Use the operator :=:= to represent assignment of equality, and :∈:∈ to represent assignment of membership (i.e., assigning the property of set membership to an object)
Create a set ArAr: 0:∈Ar[A]:∈Ar⟺A∈Ar[A1,A2]:∈Ar⟺A1,A2∈Ar0:∈Ar[A]:∈Ar⟺A∈Ar[A1,A2]:∈Ar⟺A1,A2∈Ar
Create a set Ar2Ar2: [0]:∈Ar2[A]:∈Ar2⟺A∈Ar2[A1,A2]:∈Ar2⟺A1,A2∈Ar2[0]:∈Ar2[A]:∈Ar2⟺A∈Ar2[A1,A2]:∈Ar2⟺A1,A2∈Ar2
Create a set BrBr: [A]:∈Br⟺A∈Ar2[A1,A2]:∈Br⟺A1,A2∈Br[A]:∈Br⟺A∈Ar2[A1,A2]:∈Br⟺A1,A2∈Br