proposition: Lemma col_mxblock B_ j : col j (\mxblock_(i, j) B_ i j) = \mxcol_i col (sig2 j) (B_ i (sig1 j)).
proof: Proof. by apply/colP => l; rewrite !mxE. 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)