intros a b c H'; auto.
Qed.

(* Specifications: *)
Theorem plusA_eqA_comp_r :
forall a b c : A, eqA a b -> eqA (plusA c a) (plusA c b).
Theorem eqA0_left : forall a b : A, eqA a A0 -> eqA b (plusA a b).
intros a b H'; apply eqA_trans with (y := plusA A0 b); auto.
Theorem multA_eqA_comp_r :
forall a b c : A, eqA a b -> eqA (multA c a) (multA c b).
Theorem multA_dist_r :
forall a b c : A, eqA (plusA (multA a c) (multA b c)) (multA (plusA a b) c).