proposition: Lemma addsmx_addKr n m1 m2 (A B : 'M_(m1, n)) (C : 'M_(m2, n)) : (B <= C)%MS -> ((A + B)%R + C :=: A + C)%MS.
proof: Proof. by rewrite -!(addsmxC C) addrC; apply: addsmx_addKl. 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)