namespace boost {
  unspecified throw_on_failure;
  template<typename TypeOut, typename TypeIn, typename Converter> 
    boost::optional< TypeOut > convert(TypeIn const &, Converter const &);
  template<typename TypeOut, typename TypeIn> 
    boost::optional< TypeOut > convert(TypeIn const &);

  // Boost.Convert non-optional deployment interface. 
  template<typename TypeOut, typename TypeIn, typename Converter> 
    TypeOut convert(TypeIn const & value_in, Converter const & converter, 
                    unspecified);
  template<typename TypeOut, typename TypeIn, typename Converter, 
           typename Fallback> 
    std::enable_if< is_convertible< Fallback, TypeOut >::value, TypeOut >::type 
    convert(TypeIn const & value_in, Converter const & converter, 
            Fallback const & fallback);
  template<typename TypeOut, typename TypeIn, typename Converter, 
           typename Fallback> 
    std::enable_if< cnv::is_fun< Fallback, TypeOut >::value, TypeOut >::type 
    convert(TypeIn const & value_in, Converter const & converter, 
            Fallback fallback);
  namespace cnv {
    template<typename Converter, typename TypeOut, typename TypeIn> 
      struct reference;

    template<typename Converter, typename TypeOut> 
      struct reference<Converter, TypeOut, void>;
    template<typename TypeOut, typename TypeIn, typename Converter> 
      reference< Converter, TypeOut, TypeIn > apply(Converter const &);
    template<typename TypeOut, typename Converter> 
      reference< Converter, TypeOut, void > apply(Converter const & cnv);
  }
}