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@@ -0,0 +1,18 @@

+digraph G {

+  compound=true

+

+  subgraph clusterA {

+    a1 -> a2 -> a3

+  }

+  subgraph clusterB {

+    subgraph clusterC {

+      c1 -> c2

+    }

+    b1 -> b2

+    b1 -> b3

+    b1 -> c2

+  }

+  a1 -> b2 [lhead=clusterB]

+  c2 -> a3 [ltail=clusterC]

+  b3 -> a3 [ltail=clusterB, lhead=clusterA]

+}