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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 _remove_existing(self):
 """ When a change is detected, remove keys that existed in the old file. """

        for key in self._keys:
            if key in os.environ:
                LOG.debug('%r: removing old key %r', self, key)
                del os.environ[key]
        self._keys = []