Arguments mulsmxP {m1 m2 n A R1 R2}.

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