docs: def strings(self):
          """
          Write strings sheet.
          """
  
          sheet = self.result.add_sheet("strings")
          self.header(sheet, "strings")
  
          n_row = 1  # row number
  
          for entry in self.po:
              row = sheet.row(n_row)
              row.write(0, entry.msgid)
              row.write(1, entry.msgstr)
              n_row += 1
              sheet.flush_row_data()