proposition: Lemma col_mxrow m (B_ : forall j, 'M[T]_(m, q_ j)) j : col j (\mxrow_j B_ j) = col (sig2 j) (B_ (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)