code: def _find_longest_parent_path(path_set, path):
    
    # This could likely be more efficiently implemented with a trie
    # data-structure, but we don't want to add an extra dependency for that.
    while path not in path_set:
      if not path:
        return None
      path = os.path.dirname(path)
    return path
docs: Finds the longest "parent-path" of 'path' in 'path_set'.
  
    This function takes and returns "path-like" strings which are strings
    made of strings separated by os.sep. No file access is performed here, so
    these strings need not correspond to actual files in some file-system..
    This function returns the longest ancestor path
    For example, for path_set=["/foo/bar", "/foo", "/bar/foo"] and
    path="/foo/bar/sub_dir", returns "/foo/bar".
  
    Args:
      path_set: set of path-like strings -- e.g. a list of strings separated by
        os.sep. No actual disk-access is performed here, so these need not
        correspond to actual files.
      path: a path-like string.
  
    Returns:
      The element in path_set which is the longest parent directory of 'path'.