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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(status_file=__file__):
 """Traverse directory trees to find mapreduce.yaml file. Begins with the location of status.py and then moves on to check the working directory. Args: status_file: location of status.py, overridable for testing purposes. Returns: the path of mapreduce.yaml file or None if not found. """

  checked = set()
  yaml = _find_mapreduce_yaml(os.path.dirname(status_file), checked)
  if not yaml:
    yaml = _find_mapreduce_yaml(os.getcwd(), checked)
  return yaml