docs: def add(self, path):
          """Add a path to the overlay filesytem.
  
          Any filesystem operation involving the this path or any sub-paths
          of it will be transparently redirected to temporary root dir.
  
          @path: An absolute path string.
          """
          if not path.startswith(os.sep):
              raise ValueError("Non-absolute path '{}'".format(path))
          path = path.rstrip(os.sep)
          while True:
              self._paths[path] = None
              path, _ = os.path.split(path)
              if path == os.sep:
                  break