// In header: <boost/proto/transform/arg.hpp>


template<typename Expr, typename State, typename Data> 
struct impl :  proto::transform_impl< Expr, State, Data > {
  // types
  typedef typename proto::result_of::value< Expr >::type result_type;

  // public member functions
  typename proto::result_of::value< Expr >::type 
  operator()(typename impl::expr_param, typename impl::state_param, 
             typename impl::data_param) const;
};