Definition mulsmx m1 m2 n (R1 : 'A[F]_(m1, n)) (R2 : 'A_(m2, n)) :=
(\sum_i <<R1 *m lin_mx (mulmxr (vec_mx (row i R2)))>>)%MS.
Arguments mulsmx {m1%N m2%N n%N} R1%MS R2%MS.
Local Notation "R1 * R2" := (mulsmx R1 R2) : matrix_set_scope.

Lemma mulsmx_subP m1 m2 m n
(R1 : 'A_(m1, n)) (R2 : 'A_(m2, n)) (R : 'A_(m, n)) :
reflect (forall A1 A2, A1 \in R1 -> A2 \in R2 -> A1 *m A2 \in R)
(R1 * R2 <= R)%MS.