proposition: Lemma mulNmx m n p (A : 'M[R]_(m, n)) (B : 'M_(n, p)) : - A *m B = - (A *m B).
proof: Proof. by apply/matrixP=> i k; rewrite !mxE -sumrN; under eq_bigr do rewrite mxE mulNr. 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)