proposition: Lemma mxrow_sum (J : finType) m (R_ : forall i j, 'M[V]_(m, q_ j)) (P : {pred J}) : \mxrow_j (\sum_(i | P i) R_ i j) = \sum_(i | P i) \mxrow_j (R_ i j).
proof: Proof. apply/matrixP => i j; 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)