proposition: Lemma binds_app_2 : binds x a F -> binds x a (E ++ F).
proof: Proof. clear. intros H. red in H |- *. rewrite -> in_app. auto. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.MetatheoryAtom Metalib.LibTactics