The proto::literal<> terminal wrapper, and the proto::lit() function for creating proto::literal<> wrappers.

namespace boost {
  namespace proto {
    template<typename T, typename Domain = proto::default_domain> 
      struct literal;
    template<typename T> proto::literal< T & > const lit(T &);
    template<typename T> proto::literal< T const & > const lit(T const &);
  }
}