fact: Axiom app_assoc : (l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.AssocList Metalib.MetatheoryAtom Metalib.LibTactics