SIGNAL_NETWORK_TRACKABLE

namespace boost {
  namespace signals {
    class filter_base;

    template<typename T, typename Enable = void> struct is_filter;

    template<typename T> 
      struct is_component<T, typename boost::enable_if< is_filter< T > >::type>;
    template<typename T> 
      struct get_signal<T, typename boost::enable_if< is_filter< T > >::type>;
    template<typename T> 
      struct is_filter<T, typename boost::enable_if< boost::is_base_of< filter_base, T > >::type>;
  }
}