proposition: Lemma mxtrace_scalar a : \tr a%:M = a *+ n.
proof: Proof. rewrite -diag_const_mx mxtrace_diag; under eq_bigr do rewrite mxE. by rewrite sumr_const card_ord. 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)