theorem Th9:
  (inferior_setsequence Complement A).n = ((superior_setsequence A).n)`
proof
  set B = Complement A;
  n in NAT by ORDINAL1:def 12; then
  (inferior_setsequence B).n = ((superior_setsequence Complement B ).n)`
    by SETLIM_1:30;
  hence thesis;
end;
