@MISC{ZF_COLLA.MIZ,
  AUTHOR = {Bancerek, Grzegorz},
  TITLE = {The Contraction Lemma},
  DAY = {14},
  MONTH = {April},
  YEAR = {1989},
  ADDRESS1 = {Warsaw University\\Bia{\l}ystok},
  SUMMARY = {The article includes the proof of the contraction lemma
which claims that every class in which the axiom of extensionality
is valid is isomorphic with a transitive class. In this article
the isomorphism (wrt membership relation) of two sets is defined.
It is based on \cite{MOST:1}.}}

