proposition: Lemma row_mxcol n (B_ : forall i, 'M[T]_(p_ i, n)) i : row i (\mxcol_i B_ i) = row (sig2 i) (B_ (sig1 i)).
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)