// In header: <boost/log/utility/functional/logical.hpp>

struct less {
  // types
  typedef bool result_type;
  // public member functions
  template<typename T, typename U> bool operator()(T const &, U const &) const;
  // private static functions
  template<typename T, typename U> 
    static bool op(T const &, U const &, mpl::false_ const &);
  template<typename T, typename U> 
    static bool op(T const &, U const &, mpl::true_ const &);
};