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]
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.