// In header: <boost/xpressive/regex_actions.hpp>

struct front {
  // member classes/structs/unions
  template<typename Sig> 
  struct result {
  };
  template<typename This, typename Sequence> 
  struct result<This(Sequence)> {
    // types
    typedef remove_reference< Sequence >::type                                                                                              sequence_type;
    typedef mpl::if_c< is_const< sequence_type >::value, typename sequence_type::const_reference, typename sequence_type::reference >::type type;         
  };
  // public member functions
  template<typename Sequence> 
    result< front(Sequence &)>::type operator()(Sequence &) const;
};