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

template<typename T, typename U> 
struct mem_ptr :  proto::transform< mem_ptr<T, U> > {
  // types
  typedef proto::expr< proto::tag::mem_ptr, proto::list2< T, U > >       type;         
  typedef proto::basic_expr< proto::tag::mem_ptr, proto::list2< T, U > > proto_grammar;

  // member classes/structs/unions
  template<typename Expr, typename State, typename Data> 
  struct impl :
     proto::pass_through<mem_ptr>::template impl<Expr, State, Data>
  {
  };
};