};

   ///\e 

   class FileOpenError : public IOError {

   protected:

     ExceptionMember<std::string> _file;

     mutable ExceptionMember<std::string> _message_holder;

   public:

     FileOpenError(const FileOpenError &foe) : 

       IOError(foe), _file(foe._file) {}

     ///\e 

     explicit FileOpenError(const std::string& file)

       : _file(file) {}

     ///\e 

     void file(const std::string &file) { _file.set(file); }

     /// \brief Returns the filename.

     ///

     /// Returns \e null if the filename was not specified.

     const char* file() const {

       if (_file.valid() && !_file.get().empty()) {

 	return _file.get().c_str();

       } else {

 	return 0;

       }

     }

     ///\e 

     virtual const char* what() const throw() {

       try {

 	std::ostringstream ostr;

 	ostr << exceptionName() << ": ";

 	ostr << "Cannot open file - " << file();

 	_message_holder.set(ostr.str());

       }

       catch (...) {}

       if( _message_holder.valid()) return _message_holder.get().c_str();

       return exceptionName();

     }

     virtual const char* exceptionName() const {

       return "lemon::FileOpenError";

     }

     virtual ~FileOpenError() throw() {}