fact: Variable A : Ensemble U.
imports: Coq(Ensembles Arith ArithRing Wellfounded Relations Wf_nat Finite_sets Logic.Epsilon Sets.Image Lia) ZornsLemma(Classical_Wf CountableTypes Families) hydras(MoreEpsilonIota PartialFun GRelations Prelude.More_Arith)