namespace boost {
  namespace dataflow {
    template<typename PortCategory, typename ComplementPort, 
             typename Tag = default_tag> 
      struct complemented_port_traits;
    template<typename PortTraits, typename Enable = void> 
      struct is_complemented_port_traits;
    template<typename T, typename Tag = default_tag, typename Enable = void> 
      struct is_complemented_port;
  }
}