code: def get_document_summary(self, N=None, cite_sort=True, refresh=True):
          abstracts = self.get_abstracts(refresh=refresh)
          if cite_sort:
              counts = [(a, int(a.citedby_count)) for a in abstracts]
              counts.sort(reverse=True, key=itemgetter(1))
              abstracts = [a[0] for a in counts]
          if N is None:
              N = len(abstracts)
          s = [u'{0} of {1} documents'.format(N, len(abstracts))]
          for i in range(N):
              s += ['{0:2d}. {1}\n'.format(i + 1, str(abstracts[i]))]
          return '\n'.join(s)