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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 persistent_id(self, obj):
 """ Tags objects with a persistent ID, but do NOT emit it """

        if getattr(obj, '_PERSIST_REFERENCES', None):
            objid = id(obj)
            obj._persistent_ref = objid
            _weakmemos[objid] = obj
        return None