proposition: Lemma or4_equiv: forall A1 A2 A3 A4 , or4 A1 A2 A3 A4 <-> A1 \/ A2 \/ A3 \/ A4 .
proof: Proof. intros. split. intros H. rewrite <- or3_equiv. destruct H. constructor 1. exact H. constructor 2. exact H. constructor 3. left. exact H. constructor 3. right. exact H. intros H. destruct H. constructor 1. exact H. rewrite <- or3_equiv in H. destruct H. constructor 2. exact H. constructor 3. exact H. constructor 4. exact H. Qed.
imports: Coq(Setoid Morphisms)