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

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