proposition: Lemma dlsubmx_diag m1 m2 n1 n2 (A : 'M[V]_(m1 + m2, n1 + n2)) : n1 <= m1 -> is_diag_mx A -> dlsubmx A = 0.
proof: Proof. move=> leq_m2_n2 /is_diag_mxP Adiag; apply/matrixP => i j. by rewrite !mxE Adiag// gtn_eqF//= ltn_addr// (@leq_trans n1). 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)