proposition: Lemma submxrow_matrix B_ j : submxrow (\mxblock_(i, j) B_ i j) j = \mxcol_i B_ i j.
proof: Proof. by rewrite mxblockEh mxrowK. 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)