docs: def add_label_work_count(self):
          """Adds to each result row a count of the number of works within the
          label contain that n-gram.
  
          This counts works that have at least one witness carrying the
          n-gram.
  
          This correctly handles cases where an n-gram has only zero
          counts for a given work (possible with zero-fill followed by
          filtering by maximum count).
  
          """
          self._logger.info('Adding label work count')
  
          def add_label_text_count(df):
              work_maxima = df.groupby(constants.WORK_FIELDNAME,
                                       sort=False).any()
              df.loc[:, constants.LABEL_WORK_COUNT_FIELDNAME] = work_maxima[
                  constants.COUNT_FIELDNAME].sum()
              return df
  
          if self._matches.empty:
              self._matches[constants.LABEL_WORK_COUNT_FIELDNAME] = 0
          else:
              self._matches.loc[:, constants.LABEL_WORK_COUNT_FIELDNAME] = 0
              self._matches = self._matches.groupby(
                  [constants.LABEL_FIELDNAME, constants.NGRAM_FIELDNAME],
                  sort=False).apply(add_label_text_count)
          self._logger.info('Finished adding label work count')