proposition: Lemma seq_fset_perm : perm_eq (seq_fset s) (undup s).
proof: Proof. by rewrite [seq_fset]unlock //= sort_keys_perm. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)