// In header: <boost/proto/generate.hpp>
template<typename First, typename Second> 
struct compose_generators :  proto::callable {
  // member classes/structs/unions
  template<typename This, typename Expr> 
  struct result<This(Expr)> :  
    boost::result_of<
      Second(typename boost::result_of<First(Expr)>::type)
    >
  {
  };
  // public member functions
  template<typename Expr> 
    typename boost::result_of<
      Second(typename boost::result_of<First(Expr)>::type)
    >::type 
    operator()(Expr const &) const;
};