proposition: Lemma row_mxrow m (B_ : forall j, 'M[T]_(m, q_ j)) i : row i (\mxrow_j B_ j) = \mxrow_j (row i (B_ j)).
proof: Proof. by apply/rowP => 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)