Plan 9 from Bell Labs’s /usr/web/sources/contrib/steve/root/sys/src/cmd/graphviz/graphs/directed/clust1.dot
digraph G { subgraph cluster_c0 {a0 -> a1 -> a2 -> a3;} subgraph cluster_c1 {b0 -> b1 -> b2 -> b3;} x -> a0; x -> b0; a1 -> a3; a3 -> a0; }