clust3.dot   [plain text]

digraph G {
	subgraph cluster_c0 {a0 -> a1 -> a2 -> a3;}
	subgraph cluster_c1 {b0 -> b1 -> b2 -> b3;}
	x -> a0;
	x -> b0;
	a1 -> b3;
	b1 -> a3;
}