Write a category column defined by *level* to *worksheet* at offset
        *col* and formatted with *num_format*.

def _write_cat_column(self, worksheet, col, level, num_format):
        """
        Write a category column defined by *level* to *worksheet* at offset
        *col* and formatted with *num_format*.
        """
        worksheet.set_column(col, col, 10)  # wide enough for a date
        for off, name in level:
            row = off + 1
            worksheet.write(row, col, name, num_format)