docs: def split_key(key):
      """Splits a node key."""
      if key == KEY_SEP:
          return ()
      key_chunks = tuple(key.strip(KEY_SEP).split(KEY_SEP))
      if key_chunks[0].startswith(KEY_SEP):
          return (key_chunks[0][len(KEY_SEP):],) + key_chunks[1:]
      else:
          return key_chunks