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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 __parameter_enum(self, param):
 """Returns enum descriptor of a parameter if it is an enum. An enum descriptor is a list of keys. Args: param: A simple field. Returns: The enum descriptor for the field, if it's an enum descriptor, else returns None. """

    if isinstance(param, messages.EnumField):
      return [enum_entry[0] for enum_entry in sorted(
          param.type.to_dict().items(), key=lambda v: v[1])]