proposition: Lemma app_assoc : (l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).
proof: Proof. clear. apply app_ass. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.MetatheoryAtom Metalib.LibTactics