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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 txn(self, overwrite=False, lock=True):
 """Context manager for a state modification transaction."""

        if lock:
            self._lock.acquire()
        try:
            new_state, existing_generation = self.state_and_generation
            new_state = copy.deepcopy(new_state)
            yield new_state
            if overwrite:
                existing_generation = None
            self.set_state(new_state, existing_generation=existing_generation)
        finally:
            if lock:
                self._lock.release()