fact: Definition is_diag_mx m n (A : 'M[V]_(m, n)) := [forall i : 'I__, forall j : 'I__, (i != j :> nat) ==> (A i j == 0)].
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)