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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 _prepack(self):
        '''
        Prepack stage. For parser internal use.
        '''
        current = self
        while current is not None:
            current._parser.prepack(current, skip_self = True)
            current = getattr(current, '_sub', None)
        current = self
        while current is not None:
            current._parser.prepack(current, skip_sub = True)
            current = getattr(current, '_sub', None)