<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def save_book(self, f_buf=None):
 """ Return a file buffer containing the resulting xls :param obj f_buf: A file buffer supporting the write and seek methods """

        if f_buf is None:
            f_buf = StringIO.StringIO()
        f_buf.write(openpyxl.writer.excel.save_virtual_workbook(self.book))
        f_buf.seek(0)
        return f_buf