proposition: Lemma submxrow_sum (J : finType) m (R_ : forall i, 'M[V]_(m, sq)) (P : {pred J}) j: submxrow (\sum_(i | P i) R_ i) j = \sum_(i | P i) submxrow (R_ i) j.
proof: Proof. apply/matrixP => i i'; rewrite !(mxE, summxE). by apply: eq_bigr => 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)