proposition: Lemma proj_mx_sub m n U V (W : 'M_(m, n)) : (W *m proj_mx U V <= U)%MS.
proof: Proof. by rewrite !mulmx_sub // -addsmxE addsmx0. 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)