Arguments mulsmxP {m1 m2 n A R1 R2}.

Lemma mulsmxA m1 m2 m3 n (R1 : 'A_(m1, n)) (R2 : 'A_(m2, n)) (R3 : 'A_(m3, n)) :
(R1 * (R2 * R3) = R1 * R2 * R3)%MS.