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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 missing_nodes(self):
 """The set of targets known as dependencies but not yet defined."""

        missing = set()
        for target_addr, target_attrs in self.graph.node.items():
            if 'target_obj' not in target_attrs:
                missing.add(target_addr)
        return missing