proposition: Lemma scalar_mx_sum_delta n a : a%:M = \sum_i a *: delta_mx i i :> 'M_n.
proof: Proof. by rewrite -diag_const_mx diag_mx_sum_delta; under eq_bigr do 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)