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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(self, key):
 """Return set of descendants of node named `key` in `target_graph`. Returns from cached dict if exists, otherwise compute over the graph and cache results in the dict. """

        if key not in self:
            self[key] = set(get_descendants(self._target_graph, key))
        return self[key]