namespace boost {
  namespace dataflow {
    struct tag;
    struct default_tag;
    template<typename T> struct is_tag;
    struct default_mechanism;
    namespace args {
      struct left;
      struct right;
    }
    namespace operations {
      struct connect;
      struct connect_only;
      struct disconnect;
      struct extract;
      struct disconnect_all;
    }
    namespace ports {
      struct producer;
      struct consumer;
      struct producer_consumer;
    }
  }
}