proposition: Lemma drsubmx_diag m1 m2 n1 n2 (A : 'M[V]_(m1 + m2, n1 + n2)) : m1 = n1 -> is_diag_mx A -> is_diag_mx (drsubmx A).
proof: Proof. move=> eq_m1_n1 /is_diag_mxP Adiag; apply/is_diag_mxP => i j neq_ij. by rewrite !mxE Adiag//= eq_m1_n1 eqn_add2l. 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)