proposition: Lemma in_app : List.In x (l1 ++ l2) <-> List.In x l1 \/ List.In x l2.
proof: Proof. clear. split; auto using in_or_app, in_app_or. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.MetatheoryAtom Metalib.LibTactics