how? :)

specifically, the type of the `spec` record is

```
type server = {
  callback :
    Cohttp.Connection.t ->
    ?body:Cohttp_lwt_body.contents ->
    Cohttp.Request.t ->
    (Cohttp.Response.t * Cohttp_lwt_body.t) Lwt.t;
  conn_closed:
    Cohttp.Connection.t -> unit -> unit;
}
```

doesn't the return type of `conn_closed` need to be `unit Lwt.t` if i'm to use `C.log_s`? or is there some other logging function i should use?

i get type errors if i try either
```
    let conn_closed conn_id () =
      let cid = Cohttp.Connection.to_string conn_id in
      C.log_s c (Printf.sprintf "conn %s closed\n%!" cid)
    in
```
or 
```
    let conn_closed conn_id () =
      let cid = Cohttp.Connection.to_string conn_id in
      lwt _ = C.log_s c (Printf.sprintf "conn %s closed\n%!" cid) in
      ()
    in
```

to whit:
```
       Type unit C.io = unit Lwt.t is not compatible with type unit
```