let week_of_year year month day =
  let r = call_array ~fn:"Date::Calc::Week_of_Year"
            [sv_of_int year; sv_of_int month; sv_of_int day] in
  match r with
      [week; year] -> int_of_sv week, int_of_sv year
    | _ -> failwith "Pl_Date_Calc: week_of_year: unexpected return value"