elim H'; intros H'0 H'1; clear H'; exact H'1.
elim H'; intros H'0 H'1; clear H'; exact H'0.
Qed.

(* Specifications: *)
Theorem or_commutative : forall A B : Prop, A \/ B -> B \/ A.
intros A B h; elim h; [ intro H'; clear h; try exact H' | clear h; intro H' ].