code: def _find_mapreduce_yaml(start, checked):
    
    dir = start
    while dir not in checked:
      checked.add(dir)
      for mr_yaml_name in MR_YAML_NAMES:
        yaml_path = os.path.join(dir, mr_yaml_name)
        if os.path.exists(yaml_path):
          return yaml_path
      dir = os.path.dirname(dir)
    return None
docs: Traverse the directory tree identified by start until a directory already
    in checked is encountered or the path of mapreduce.yaml is found.
  
    Checked is present both to make loop termination easy to reason about and so
    that the same directories do not get rechecked.
  
    Args:
      start: the path to start in and work upward from
      checked: the set of already examined directories
  
    Returns:
      the path of mapreduce.yaml file or None if not found.