proposition: Lemma scalar_mxC m n a (A : 'M[R]_(m, n)) : A *m a%:M = a%:M *m A.
proof: Proof. rewrite -!diag_const_mx mul_mx_diag mul_diag_mx. by apply/matrixP => i j; rewrite !mxE mulrC. 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)