How to do links to other pages?

Examples:

(* Link to a service without parameter: *)
Html.D.a ~service:coucou [txt "coucou"] ();

(* Link to a service with parameters of type (int * (int * string)): *)
Html.D.a ~service:coucou_params
  [txt "coucou_params"] (42, (22, "ciao"));

(* Link to an external service: *)
Html.D.a
  (Eliom_service.extern
    ~prefix:"http://fr.wikipedia.org"
    ~path:["wiki";""]
    ~meth:(Eliom_service.Get Eliom_parameter.(suffix (all_suffix "suff")))
    ())
  [txt "OCaml on wikipedia"]
  ["OCaml"];

(* Low level interface for links: *)
Html.F.Raw.a
  ~a:[a_href (Xml.uri_of_string "http://en.wikipedia.org/wiki/OCaml")]
  [txt "OCaml on wikipedia"]