proposition: Lemma perm_eq_Freeg s1 s2 : perm_eq s1 s2 -> [freeg s1] = [freeg s2].
proof: Proof. move=> peq; apply/eqP/freeg_eqP=> k. by rewrite !coeff_Freeg; apply: precoeff_perm_eq. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)