code: def normalized_term_frequency(self, term, smooth=0.0):
          
          frequency = self.term_frequency(term) / self._max_frequency
          return smooth + (1.0 - smooth)*frequency
docs: Returns normalized frequency of term in document.
          http://nlp.stanford.edu/IR-book/html/htmledition/maximum-tf-normalization-1.html
  
          :parameter float smooth:
              0.0 <= smooth <= 1.0, generally set to 0.4, although some
              early work used the value 0.5. The term is a smoothing term
              whose role is to damp the contribution of the second term.
              It may be viewed as a scaling down of TF by the largest TF
              value in document.
          :returns float:
              0.0 <= frequency <= 1.0, where 0 means no occurence in document
              and 1 the most frequent term in document.