proposition: Lemma sort_keys_nil : sort_keys [::] = [::] :> seq K.
proof: Proof. have := sort_keysE ([::] : seq K). by case: sort_keys => //= a l /(_ a); rewrite mem_head. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)