+class usage_error: public std::runtime_error
+{
+private:
+ std::string help_;
+
+public:
+ usage_error(const std::string &w, const std::string &h = std::string()): std::runtime_error(w), help_(h) { }
+ ~usage_error() throw() { }
+
+ const char *help() const throw() { return help_.c_str(); }
+};
+
+