proposition: Lemma col_mxcol n (B_ : forall i, 'M[T]_(p_ i, n)) j : col j (\mxcol_i B_ i) = \mxcol_i (col j (B_ i)).
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)