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 addsmx_idPl : reflect (A + B :=: A)%MS (B <= A)%MS.
End AddsmxSub.