proposition: Lemma ursubmx_trig m1 m2 n1 n2 (A : 'M[V]_(m1 + m2, n1 + n2)) : m1 <= n1 -> is_trig_mx A -> ursubmx A = 0.
proof: Proof. move=> leq_m1_n1 /is_trig_mxP Atrig; apply/matrixP => i j. by rewrite !mxE Atrig//= ltn_addr// (@leq_trans m1). 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)