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

template<typename Expr, typename State, typename Data> 
struct transform_impl {
  // types
  typedef typename boost::remove_reference<Expr const>::type  expr;       
  typedef typename boost::add_reference<Expr const>::type     expr_param; 
  typedef typename boost::remove_reference<State const>::type state;      
  typedef typename boost::add_reference<State const>::type    state_param;
  typedef typename boost::remove_reference<Data const>::type  data;       
  typedef typename boost::add_reference<Data const>::type     data_param; 
};