proposition: Lemma row_row_mx m n1 n2 i0 (A1 : 'M_(m, n1)) (A2 : 'M_(m, n2)) : row i0 (row_mx A1 A2) = row_mx (row i0 A1) (row i0 A2).
proof: Proof. by apply/matrixP=> i j /[!mxE]; case: (split j) => j' /[1!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)