proposition: Lemma in_app_iff : List.In x (l1 ++ l2) <-> List.In x l1 \/ List.In x l2.
proof: Proof. clear. split; auto using List.in_or_app, List.in_app_or. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Coq.Strings.String String Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.CoqFSetInterface Coq.Classes.Equivalence Coq.Classes.EquivDec Metalib.CoqEqDec