apply eqA_trans with (y := plusA a A0); auto.
apply eqA_trans with (y := plusA A0 a); auto.
apply eqA_trans with (y := plusA (plusA (invA a) (invA (invA a))) a); auto.
apply eqA_trans with (y := plusA (plusA (invA (invA a)) (invA a)) a); auto.
apply eqA_trans with (y := plusA (invA (invA a)) (plusA (invA a) a)); auto.
apply eqA_trans with (y := plusA (invA (invA a)) (plusA a (invA a))); auto.
apply eqA_trans with (y := plusA (invA (invA a)) A0); auto.
Qed.

(* Specifications: *)
Theorem multA_eqA_comp_l :
forall a b c : A, eqA a b -> eqA (multA a c) (multA b c).