proposition: Lemma is_perm_mxP n (A : 'M_n) : reflect (exists s, A = perm_mx s) (is_perm_mx A).
proof: Proof. by apply: (iffP existsP) => [] [s /eqP]; exists s. 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)