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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 _get_previous_open_tag(self, obj):
 """ Return the open tag of the previous sibling """

        prev_instance = self.get_previous_instance(obj)
        if prev_instance and prev_instance.plugin_type == self.__class__.__name__:
            return prev_instance.glossary.get('open_tag')