proposition: Lemma binds_app_3 : binds x a (E ++ F) -> binds x a E \/ binds x a F.
proof: Proof. clear. induction E as [ | a1 E' ]. auto. unfold binds. simpl. intros [H | H]. inversion H; subst. auto. unfold binds in *. apply IHE' in H. intuition. Qed.
imports: Coq.FSets.FSets Coq.Lists.List Coq.Logic.Decidable Metalib.CoqFSetDecide Metalib.CoqListFacts Metalib.LibTactics Metalib.MetatheoryAtom Metalib.LibTactics