===== .ml

  let printf : string -> int -> int -> ('a, unit, string, unit) format4 -> 'a =
    let start_time = Sys.time () in
    (fun (brick_name : string) (brick_verb:int) (verb:int) (message : ('a, unit, string, unit) format4) ->
     Printf.ksprintf
       (fun s ->
    if true
    then Printf.printf "[%s][%.3f]%s" brick_name start_time s else ())
       message)

===== .mli

      val printf :
        string -> int -> int -> ('a, unit, string, unit) format4 -> 'a

===== Erro :
Error: The implementation PhTools.eliom
       does not match the interface _server/PhTools.cmi:
       ...
       In module Debug:
       Values do not match:
         val printf :
           string -> int -> int -> (unit, unit, string, unit) format4 -> unit
       is not included in
         val printf :
           string -> int -> int -> ('a, unit, string, unit) format4 -> 'a