code: private Zxid getNextProposedZxid() {
      if (lastProposedZxid.getEpoch() != establishedEpoch) {
        lastProposedZxid = new Zxid(establishedEpoch, -1);
      }
      lastProposedZxid = new Zxid(establishedEpoch,
                                  lastProposedZxid.getXid() + 1);
      return lastProposedZxid;
    }
docs: Gets next proposed zxid for leader in broadcasting phase.
  
  @return the zxid of next proposed transaction.