proposition: Lemma is_trig_mxP m n (A : 'M[V]_(m, n)) : reflect (forall i j : 'I__, (i < j)%N -> A i j = 0) (is_trig_mx A).
proof: Proof. by apply: (iffP 'forall_'forall_implyP) => /(_ _ _ _)/eqP. 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)