code: def index(self, attr):
          
  
          for i, paper in self.indexed_papers.iteritems():
              self.index_paper_by_attr(paper, attr)
docs: Indexes the :class:`.Paper`\s in this :class:`.Corpus` instance
          by the attribute ``attr``.
  
          New indices are added to :attr:`.indices`\.
  
          Parameters
          ----------
          attr : str
              The name of a :class:`.Paper` attribute.