code: def index_feature(self, feature_name, tokenize=lambda x: x, structured=False):
          
          self._init_featureset(feature_name, structured=structured)
  
          for paper in self.papers:
              self.index_paper_by_feature(paper, feature_name, tokenize, structured)
docs: Creates a new :class:`.FeatureSet` from the attribute ``feature_name``
          in each :class:`.Paper`\.
  
          New :class:`.FeatureSet`\s are added to :attr:`.features`\.
  
          Parameters
          ----------
          feature_name : str
              The name of a :class:`.Paper` attribute.