// In header: <boost/proto/traits.hpp>

template<typename Expr, typename State, typename Data> 
struct impl :  proto::transform_impl< Expr, State, Data > {
  // types
  typedef Expr result_type;
  // public member functions
  Expr operator()(typename impl::expr_param, typename impl::state_param, 
                  typename impl::data_param) const;
};