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