How do I specify that I want xxx to be HM.t ?

##############################
# scanner.mli

type my_header =
{
     ...
}

val get_leaves : my_header xxx -> string list

#################################
# scanner.ml

module HM = Map.Make(String)

(* val get_leaves : my_header HM.t -> string list  *)
let get_leaves headers =
  ...