intros a b c; apply eqA_trans with (y := plusA (multA c a) (multA c b)); auto.
apply eqA_trans with (y := multA c (plusA a b)); auto.
Qed.

(* Specifications: *)
Theorem multA_A0_r : forall a : A, eqA (multA a A0) A0.
intros a; apply eqA_trans with (y := multA A0 a); auto.