code: func bucketIndexKey(parentID [4]byte, key []byte) []byte {
  	// The serialized bucket index key format is:
  	//   <bucketindexprefix><parentbucketid><bucketname>
  	indexKey := make([]byte, len(bucketIndexPrefix)+4+len(key))
  	copy(indexKey, bucketIndexPrefix)
  	copy(indexKey[len(bucketIndexPrefix):], parentID[:])
  	copy(indexKey[len(bucketIndexPrefix)+4:], key)
  	return indexKey
  }
docs: // bucketIndexKey returns the actual key to use for storing and retrieving a
  // child bucket in the bucket index.  This is required because additional
  // information is needed to distinguish nested buckets with the same name.