proposition: Lemma addmx_sub m1 m2 n (A : 'M_(m1, n)) (B : 'M_(m1, n)) (C : 'M_(m2, n)) : (A <= C)%MS -> (B <= C)%MS -> ((A + B)%R <= C)%MS.
proof: Proof. by case/submxP=> A' ->; case/submxP=> B' ->; rewrite -mulmxDl submxMl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype finfun bigop finset fingroup perm order) mathcomp(div prime binomial ssralg finalg zmodp matrix)