code: def _required_args(fn):
    
    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]
docs: Returns arguments of fn with default=REQUIRED_ARG.