proposition: Lemma row_mxblock B_ i : row i (\mxblock_(i, j) B_ i j) = \mxrow_j row (sig2 i) (B_ (sig1 i) 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)