theory Prelude
imports Main
begin

(*
To make the theory work under Isabelle 2009 and 2011

Isabelle 2009: set_ext
Isabelle 2011: set_eqI

*)


lemma set_eq_intro:
  "(\<And>x. (x \<in> A) = (x \<in> B)) \<Longrightarrow> A = B"
by blast


end