@MISC{CLASSES2.MIZ,
  AUTHOR = {Nowak, Bogdan and Bancerek, Grzegorz},
  TITLE = {Universal Classes},
  DAY = {10},
  MONTH = {April},
  YEAR = {1990},
  ADDRESS1 = {{\L}\'od\'z University},
  ADDRESS2 = {Warsaw University\\Bia{\l}ystok},
  SUMMARY = {In the article we have shown that there exist universal classes, i.e.
there are sets which are closed w.r.t. basic set theory operations.}}

