exception Chosen of string

(* h : (string, string list) Hashtbl.t *)

let choose h =
  let tot = Hashtbl.fold (fun _ v s -> s + List.length v) h 0 in
  try
    ignore
      (Hashtbl.fold
        (fun k v r ->
          let len = List.length v in
          let t = r - len in
          if t < 0 then raise (Chosen k)
          else t)
        h
        (Random.int tot));
    assert false (* not reached *)
  with Chosen k ->
    k