docs: def add_label_count(self):
          """Adds to each result row a count of the number of occurrences of
          that n-gram across all works within the label.
  
          This count uses the highest witness count for each work.
  
          """
          self._logger.info('Adding label count')
  
          def add_label_count(df):
              # For each n-gram and label pair, we need the maximum count
              # among all witnesses to each work, and then the sum of those
              # across all works.
              work_maxima = df.groupby(constants.WORK_FIELDNAME,
                                       sort=False).max()
              df.loc[:, constants.LABEL_COUNT_FIELDNAME] = work_maxima[
                  constants.COUNT_FIELDNAME].sum()
              return df
  
          if self._matches.empty:
              self._matches[constants.LABEL_COUNT_FIELDNAME] = 0
          else:
              self._matches.loc[:, constants.LABEL_COUNT_FIELDNAME] = 0
              self._matches = self._matches.groupby(
                  [constants.LABEL_FIELDNAME, constants.NGRAM_FIELDNAME],
                  sort=False).apply(add_label_count)
          self._logger.info('Finished adding label count')