proposition: Lemma is_diag_mxP m n (A : 'M[V]_(m, n)) : reflect (forall i j : 'I__, i != j :> nat -> A i j = 0) (is_diag_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)