code: def is_pp_seq_no_stable(self, msg: Checkpoint):
          
          pp_seq_no = msg.seqNoEnd
          ck = self.firstCheckPoint
          if ck:
              _, ckState = ck
              return ckState.isStable and ckState.seqNo >= pp_seq_no
          else:
              return False
docs: :param ppSeqNo:
          :return: True if ppSeqNo is less than or equal to last stable
          checkpoint, false otherwise