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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 add_back_ref(self, back_ref, attr=None):
 """Add reference from back_ref to self :param back_ref: back_ref to add :type back_ref: Resource :rtype: Resource """

        back_ref.add_ref(self, attr)
        return self.fetch()