fact: Definition Countable_Union (A: Type) (P: nat -> Ensemble A) : Ensemble A := fun x => exists i, P i x.
imports: Coq.Classes.Morphisms Coq.Classes.Equivalence Coq.Sets.Ensembles Coq.Sets.Constructive_sets CertiGraph.lib.Coqlib CertiGraph.lib.EquivDec_ext