code: def add_sequence(self, words):
          
          n = self.n
          words = ['',] * (n-1) + words
          for i in range(len(words)-n):
              self.add(tuple(words[i:i+n]))
docs: Add each of the tuple words[i:i+n], using a sliding window.
          Prefix some copies of the empty word, '', to make the start work.