proposition: Lemma ulsubmx_diag m1 m2 n1 n2 (A : 'M[V]_(m1 + m2, n1 + n2)) : is_diag_mx A -> is_diag_mx (ulsubmx A).
proof: Proof. rewrite !is_diag_mxEtrig trmx_ulsub. by move=> /andP[/ulsubmx_trig-> /ulsubmx_trig->]. 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)