docs: def metadata(self):
          """
          Write metadata sheet.
          """
  
          sheet = self.result.add_sheet("metadata")
          self.header(sheet, "metadata")
  
          n_row = 1  # row number
  
          for k in self.po.metadata:
              row = sheet.row(n_row)
              row.write(0, k)
              row.write(1, self.po.metadata[k])
              n_row += 1
              sheet.flush_row_data()