proposition: Lemma mxrowK m B_ j : @submxrow m (\mxrow_j B_ j) j = B_ j.
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)