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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 begin(self, access_mode=None):
 """ Begins a new transaction, raises SystemError exception if a transaction is in progress """

        if self._active_transaction:
            raise SystemError("Transaction in progress")
        self._active_transaction = self.driver.session(access_mode=access_mode).begin_transaction()