(* Proof of lem_or_sym                                                     *)
elim H; clear H; intros.
right; assumption.
left; assumption.
Qed.
(* End of proof of lem_or_sym                                              *)

(* Specifications: *)
Lemma lem_and_sym : forall a b : Prop, a /\ b -> b /\ a.
intros; tauto.