#include <boost/phoenix.hpp>
#include <boost/fusion/adapted/std_tuple.hpp>
#include <boost/phoenix/fusion/at.hpp>

#include <iostream>
#include <valarray>
#include <tuple>

int main() {
   using boost::phoenix::arg_names::arg1 ;
   using boost::phoenix::arg_names::arg2 ;
   using boost::phoenix::val ;
   using boost::phoenix::at_c ;

   double mass(1.) ;
   auto apply(boost::phoenix::at_c<0>(arg1) 
              += boost::phoenix::at_c<1>(arg1) / val(mass) * arg2 ) ;
   std::valarray<double> item_a = { 1., 1., 1. } ;
   std::valarray<double> item_b = { 1., 1., 1. } ;
   auto test(std::make_tuple(item_a, item_b)) ;

   for (size_t i(0); i<std::get<0>(test).size(); ++i)
      std::cout << std::get<0>(test)[i] << " ";
   std::cout << std::endl ;
   for (size_t i(0); i<std::get<1>(test).size(); ++i)
      std::cout << std::get<0>(test)[i] << " ";
   std::cout << std::endl ;
   std::cout << "======================================" << std::endl ;
   apply (test, 1.) ;
   
   std::get<0>(test) += std::get<0>(test) / mass * 1. ;   
   for (size_t i(0); i<std::get<0>(test).size(); ++i)
      std::cout << std::get<0>(test)[i] << " ";
   std::cout << std::endl ;
   for (size_t i(0); i<std::get<1>(test).size(); ++i)
      std::cout << std::get<1>(test)[i] << " ";
   std::cout << std::endl ;
}