proposition: Lemma mxcolK n B_ i : @submxcol n (\mxcol_i B_ i) i = B_ i.
proof: Proof. apply/matrixP => k l; rewrite !mxE !Rank2K. by do !case: _ / esym; rewrite !cast_ord_id. 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 countalg finalg zmodp)