code: def score(self, plaintext):
          "Return a score for text based on how common letters pairs are."
          s = 1.0
          for bi in bigrams(plaintext):
              s = s * self.P2[bi]
          return f score(self, plaintext):
          "Return a score for text based on how common letters pairs are."
          s = 1.0
          for bi in bigrams(plaintext):
              s = s * self.P2[bi]
          return s
docs: Return a score for text based on how common letters pairs are.