reserve U for Universe;
reserve x for Element of U;

theorem
  EvenNAT is class of FinSETS
  proof
    now
      EvenNAT c= NAT & omega c= FinSETS by CLASSES4:43;
      hence EvenNAT c= FinSETS;
      thus EvenNAT is denumerable by Th3;
    end;
    hence thesis by Th31;
  end;
