namespace boost {
  namespace dataflow {
    template<typename PortCategory, typename PortSequence, 
             typename Tag = default_tag> 
      struct vector_port_traits;
    template<typename Traits, typename Enable = detail::enable_guard> 
      struct is_vector_port_traits;
    template<typename T, typename Tag = default_tag, typename Enable = void> 
      struct is_vector_port;
  }
}