class time_duration {
public:
  // types
  typedef time_res_traits                          rep_type;               
  typedef time_res_traits::day_type                day_type;               
  typedef time_res_traits::hour_type               hour_type;              
  typedef time_res_traits::min_type                min_type;               
  typedef time_res_traits::sec_type                sec_type;               
  typedef time_res_traits::fractional_seconds_type fractional_seconds_type;
  typedef time_res_traits::tick_type               tick_type;              
  typedef time_res_traits::impl_type               impl_type;              

  // construct/copy/destruct
  time_duration(hour_type, min_type, sec_type, fractional_seconds_type = 0);
  time_duration();
  time_duration(boost::date_time::special_values);
  time_duration(impl_type);

  // public member functions

  // private member functions
};