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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _find_mapreduce_yaml(start, checked):
 """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. """

  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