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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 _list_fields(self):
 """ List the fields stored in the model, including data, model, and training options. Each field can be queried with the ``get`` method. Returns ------- out : list List of fields queryable with the ``get`` method. """

        opts = {'model': self.__proxy__, 'model_name': self.__name__}
        response = _turicreate.extensions._nearest_neighbors.list_fields(opts)

        return sorted(response.keys())