namespace boost {
  namespace signals {
    template<typename Input, typename Output, typename Enable = void> 
      struct connect_impl;
    template<typename Input, typename Output> void connect(Input &, Output &);
    template<typename Input, typename Output> 
      void connect(Input &, const Output &);
    template<typename Input, typename Output> 
      void connect(const Input &, Output &);
    template<typename Input, typename Output> 
      void connect(const Input &, const Output &);
  }
}