code: func createCompositeKeyForPurgeIndexByTxid(txid string, uuid string, blockHeight uint64) []byte {
  	var compositeKey []byte
  	compositeKey = append(compositeKey, purgeIndexByTxidPrefix)
  	compositeKey = append(compositeKey, compositeKeySep)
  	compositeKey = append(compositeKey, createCompositeKeyWithoutPrefixForTxid(txid, uuid, blockHeight)...)
  
  	return compositeKey
  }
docs: // createCompositeKeyForPurgeIndexByTxid creates a key to index private write set based on
  // txid such that purge based on txid can be achieved. The structure
  // of the key is <purgeIndexByTxidPrefix>~txid~uuid~blockHeight.