#include <boost/proto/core.hpp>
#include <boost/proto/debug.hpp>
#include <boost/proto/transform.hpp>

using namespace boost;
using namespace boost::proto;

struct vec
    : or_<
          when<
              subscript< plus<vec,vec>, terminal<_> >
            , functional::make_expr<tag::plus>(
                  functional::make_expr<tag::subscript>(
                      _left(_left), _right
                  ),
                  functional::make_expr<tag::subscript>(
                      _right(_left), _right
                  )
              )
          >
        , plus< vec, vec >
        , terminal< _ >
      >
{};
 
struct arg1_tag {};
terminal<arg1_tag>::type const arg1 = {};
struct arg2_tag {};
terminal<arg2_tag>::type const arg2 = {};

int main()
{
    display_expr((arg1+arg2+arg1)[1]);
    display_expr(vec()((arg1+arg2+arg1)[1]));
}