fact: Axiom in_app_iff : List.In x (l1 ++ l2) <-> List.In x l1 \/ List.In x l2.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.AssocList Metalib.MetatheoryAtom Metalib.LibTactics