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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 _trim(self):
 """ Removes oldest backups that exceed the limit configured in backup_count option. Does not write back to file system, make sure to call self._write() afterwards. """

        index = self._get_index()
        backup_limit = config().backup_count() - 1

        for changeset in index[backup_limit:]:
            self.delete(changeset[0], p_write=False)