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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 _required_args(fn):
 """Returns arguments of fn with default=REQUIRED_ARG."""

  spec = getargspec(fn)
  if not spec.defaults:
    return []

  arg_names = spec.args[-len(spec.defaults):]
  return [name for name, val in zip(arg_names, spec.defaults)
          if val is REQUIRED_ARG]