proposition: Lemma equiv_symm: forall L1 L2, Equiv L1 L2 <-> Equiv L2 L1.
proof: Proof. unfold Equiv; split; intros. - rewrite H. intuition. - rewrite H. intuition. Qed.
imports: Coq.Setoids.Setoid Coq.Bool.Bool Coq.Logic.Classical_Prop