How to set Cookie with Opium?

I try to use Cookie with Opium.
sample code explains how to use Cookie module.

Therefore i wrote a following code.

let test = get "/auth/test" begin fun (_req: Request.t) ->
    `String "aaa"
    |> respond
    |> Cookie.set ~key:"something" ~data:"value"
    |> Lwt.return

But when i access to that URL, Cookie does not set.

[koji:hello_opium]$ curl -i  http://localhost:3000/auth/test
HTTP/1.1 200 OK
access-control-allow-headers: *
access-control-allow-methods: GET,POST,PUT,DELETE,OPTIONS
access-control-allow-origin: http://localhost:3000
content-length: 3


Do i miss something?

I found solution.
It was my misunderstand…

That code is correct.
Further it requires Cookie.m middleware to set Cookie to header.

let app =
  App.empty |> middleware Cookie.m |> test
let _ = app |> App.run_command