code: func base(key []byte, offset int) ([]byte, error) {
  	parts := bytes.Split(key, []byte{backend.Separator})
  	if len(parts) < offset+1 {
  		return nil, trace.NotFound("failed parsing %v", string(key))
  	}
  	return parts[len(parts)-offset-1], nil
  }
docs: // base returns last element delimited by separator, index is
  // is an index of the key part to get counting from the end