reserve U for Universe;
reserve x for Element of U;
reserve U1,U2 for Universe;

theorem
  ex X being set st union X is class of FinSETS & not X is class of FinSETS &
    not X is Set of FinSETS & X is Set of SETS
  proof
    take {NAT};
    union {NAT} is FinSETS-Class & not {NAT} is FinSETS-Class &
      not {NAT} is FinSETS-set & {NAT} is SETS-set
      by CLASSES4:72,CLASSES2:57,Th17;
    hence thesis by Def10,Def12;
  end;
