To be or not to be - these are the options for Option.map

Ashish Agarwal showed me how to neatly find source files for OPAM packages. This allowed me to locate the .mli file for Core.Std.Option.map:
 

 ~/.opam/system/lib/core_kernel/option.mli
 

 Using this command:
 

 opam config var core_kernel:lib


 -----------------
 

 I have this code that uses Core.Std.Option.map:
 

 let handler ~body:_ _sock req =
  let uri = Cohttp.Request.uri req in
  match Uri.path uri with 
  | "/test" ->
       Uri.get_query_param uri "hello"
    |> Core.Std.Option.map ~f:(fun v -> "hello: " ^ v)
    |> Core.Std.Option.value ~default:"No param hello supplied"
    |> Cohttp_async.Server.respond_with_string
  | _ ->
    Cohttp_async.Server.respond_with_string ~code:`Not_found "Route not found"
 

 However, looking in ~/.opam/system/lib/core_kernel/option.mli reveals no definition of the map function. The closest match is "val map2 : 'a t -> 'b t -> f:('a -> 'b -> 'c) -> 'c t".
 

 How can the code compile just fine when the 'map' definition isn't there?
 

 This is the content of  ~/.opam/system/lib/core_kernel/option.mli:
 https://github.com/janestreet/core_kernel/blob/master/lib/option.mli https://github.com/janestreet/core_kernel/blob/master/lib/option.mli