Let eqmx_sum_nop m n (A : 'M_(m, n)) : (addsmx_nop A :=: A)%MS.
Section AddsmxSub.
Variable (m1 m2 n : nat) (A : 'M[F]_(m1, n)) (B : 'M[F]_(m2, n)).

Lemma adds0mx_id m1 n (B : 'M_n) : ((0 : 'M_(m1, n)) + B)%MS = B.
Canonical addsmx_monoid n :=
Monoid.Law (@addsmxA n n n n) (@adds0mx_id n n) (@addsmx0_id n n).