Arguments mulsmx_subP {m1 m2 m n R1 R2 R}.

Lemma muls_eqmx m1 m2 m3 m4 n (R1 : 'A_(m1, n)) (R2 : 'A_(m2, n))
(R3 : 'A_(m3, n)) (R4 : 'A_(m4, n)) :
(R1 :=: R3 -> R2 :=: R4 -> R1 * R2 = R3 * R4)%MS.