docs: def _writeFeatures(self, i, image):
          """
          Write a text file containing the features as a table.
  
          @param i: The number of the image in self._images.
          @param image: A member of self._images.
          @return: The C{str} features file name - just the base name, not
              including the path to the file.
          """
          basename = 'features-%d.txt' % i
          filename = '%s/%s' % (self._outputDir, basename)
          featureList = image['graphInfo']['features']
          with open(filename, 'w') as fp:
              for feature in featureList:
                  fp.write('%s\n\n' % feature.feature)
          return basename