Nested subgraphs are flattened

[coder0xff]

A graph such as

digraph G {
	subgraph cluster_0 {
		subgraph cluster_1 {
			A;
			B;
			A -> B;
		}
	}
}

Does not preserve the subgraph hierarchy, instead making two subgraphs at the top level. DotSubGraph has no field for child subgraphs.