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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 _start_callbacks(self, result, exception):
 """Perform the callback chain going back and forth between the callback and errback as needed. If an exception is raised and the entire chain is gone through without a valid errback then its simply logged. """

        if self._cancelled:
            raise CancelledError()
        if self.called:
            raise AlreadyCalledError()

        self._result = result
        self._exception = exception
        if self._exception:
            self._tb_info = ''.join(traceback.format_tb(sys.exc_info()[2])) 
        self.called = True
        self._do_callbacks()