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A graph is returned so that you do not need to erase all the objects passed back, just like in the other purge().
A graph is returned so that you do not need to erase all the objects passed back, just like in the other purge(). However, if you want to selectively erase only part of the objects passed back, you must only erase root-type nodes on the graph. In other words, from the above example, the graph passed back would contain both the Layer and Linetype nodes, but there would be an edge from the Layer to the Linetype. Thus only the Layer would be a root-type node, with no incoming edges. That means that you could erase the Layer by itself, but not the Linetype. If you want to erase the Linetype, then you must also erase the Layer. That's why the return data is in a graph.