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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 extract_tokens(self, input_col='result', output_col='tokens'):
 """ Extracts the tokens from UAST nodes. :param input_col: column containing the list of nodes to extract tokens from :type input_col: str :param output_col: column to place the resultant tokens :type output_col: str :rtype: UASTsDataFrame """

        return UASTsDataFrame(self._engine_dataframe.extractTokens(input_col, output_col),
                              self._session, self._implicits)