fun number_in_month_tailrec(lod0: (int * int * int), month: int) =
  let
    fun fn_for_lod(lod: (int * int * int), acc: int) =
      if null lod
      then acc
      else
        if #2 (hd lod) = month
        then fn_for_lod(tl lod, acc + 1)
        else fn_for_lod(tl lod, acc)
  in
    fn_for_lod(lod0, 0)
  end