code: func prepareExpiryEntries(committingBlk uint64, dataEntries []*dataEntry, missingDataEntries map[missingDataKey]*bitset.BitSet,
  	btlPolicy pvtdatapolicy.BTLPolicy) ([]*expiryEntry, error) {
  
  	var expiryEntries []*expiryEntry
  	mapByExpiringBlk := make(map[uint64]*ExpiryData)
  
  	// 1. prepare expiryData for non-missing data
  	for _, dataEntry := range dataEntries {
  		prepareExpiryEntriesForPresentData(mapByExpiringBlk, dataEntry.key, btlPolicy)
  	}
  
  	// 2. prepare expiryData for missing data
  	for missingDataKey := range missingDataEntries {
  		prepareExpiryEntriesForMissingData(mapByExpiringBlk, &missingDataKey, btlPolicy)
  	}
  
  	for expiryBlk, expiryData := range mapByExpiringBlk {
  		expiryKey := &expiryKey{expiringBlk: expiryBlk, committingBlk: committingBlk}
  		expiryEntries = append(expiryEntries, &expiryEntry{key: expiryKey, value: expiryData})
  	}
  
  	return expiryEntries, nil
  }
docs: // prepareExpiryEntries returns expiry entries for both private data which is present in the committingBlk
  // and missing private.