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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 master_ref(self):
 """ Filters the current DataFrame to only contain those rows whose reference is master. :rtype: ReferencesDataFrame """

        return ReferencesDataFrame(self._engine_dataframe.getMaster(),
                                   self._session, self._implicits)
        return self.ref('refs/heads/master')