<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def write(self, file_or_filename):
 """ Writes case data to file in Excel format. """

        self.book = Workbook()
        self._write_data(None)
        self.book.save(file_or_filename)