code: def frequency_model( self ):
          
          prev_letter = None
          # use a generator in corpus
          prev_letter = list(self.corpus.next_tamil_letter())[0]
          for next_letter in self.corpus.next_tamil_letter():
              # update frequency from corpus
              key = prev_letter+next_letter
              val = self.bigram.get(key,None)
              prev_letter = next_letter
              if not val:
                  self.bigram[key] = 0
              self.bigram[key] += 1
          
          return
docs: build a letter frequency model for Tamil letters from a corpus