Набор данных присоединяется к операции.

namespace boost {
  namespace unit_test {
    namespace data {
      namespace monomorphic {
        template<typename DataSet1, typename DataSet2> class join;
        template<typename DataSet1, typename DataSet2> 
          struct is_dataset<join< DataSet1, DataSet2 >>;
        template<typename DataSet1, typename DataSet2> 
          boost::lazy_enable_if_c< is_dataset< DataSet1 >::value &&is_dataset< DataSet2 >::value, result_of::join< mpl::identity< DataSet1 >, mpl::identity< DataSet2 > >>::type 
          operator+(DataSet1 && ds1, DataSet2 && ds2);
        template<typename DataSet1, typename DataSet2> 
          boost::lazy_enable_if_c< is_dataset< DataSet1 >::value &&!is_dataset< DataSet2 >::value, result_of::join< mpl::identity< DataSet1 >, data::result_of::make< DataSet2 > >>::type 
          operator+(DataSet1 && ds1, DataSet2 && ds2);
        template<typename DataSet1, typename DataSet2> 
          boost::lazy_enable_if_c<!is_dataset< DataSet1 >::value &&is_dataset< DataSet2 >::value, result_of::join< data::result_of::make< DataSet1 >, mpl::identity< DataSet2 > >>::type 
          operator+(DataSet1 && ds1, DataSet2 && ds2);
        namespace result_of {
          template<typename DataSet1Gen, typename DataSet2Gen> struct join;
        }
      }
    }
  }
}