proposition: Lemma is_diag_mx_is_trig m n (A : 'M[V]_(m, n)) : is_diag_mx A -> is_trig_mx A.
proof: Proof. by move=> /is_diag_mxP A_eq0; apply/is_trig_mxP=> i j lt_ij; rewrite A_eq0// ltn_eqF. 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)