code: func createCompositeKeyForPurgeIndexByHeight(blockHeight uint64, txid string, uuid string) []byte {
  	var compositeKey []byte
  	compositeKey = append(compositeKey, purgeIndexByHeightPrefix)
  	compositeKey = append(compositeKey, compositeKeySep)
  	compositeKey = append(compositeKey, util.EncodeOrderPreservingVarUint64(blockHeight)...)
  	compositeKey = append(compositeKey, compositeKeySep)
  	compositeKey = append(compositeKey, []byte(txid)...)
  	compositeKey = append(compositeKey, compositeKeySep)
  	compositeKey = append(compositeKey, []byte(uuid)...)
  
  	return compositeKey
  }
docs: // createCompositeKeyForPurgeIndexByHeight creates a key to index private write set based on
  // received at block height such that purge based on block height can be achieved. The structure
  // of the key is <purgeIndexByHeightPrefix>~blockHeight~txid~uuid.