Variable T : Traversal.
(* Definition liftT a b (f : a -> b) := traverse (t:=T) (F:=IdApplicative) f.  Lemma free_traversal_b a b b' (h : b -> b') (F : Applicative) (f : a -> F b) (x : T a) : map (liftT h) (traverse f x) = traverse (fun y => map h (f y)) x. *)
Let traversal_coalg a b := traverse (t:=T) (F:=KSApplicative a b) (impure (a:=a) b).

Lemma traversal_coalg_extract a (x: T a) : extractIT _ _ (traversal_coalg _ x) = x.