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+digraph G {

+  label = "label demo";

+  labeljust="center";

+  subgraph cluster0 {

+    a;

+    b;

+    a->b;

+    label = "cluster0";

+    labelloc="bottom"

+  }

+  subgraph cluster1 {

+    c;

+    d;

+    c->d;

+    label = "cluster1";

+    labelloc="top"

+  }

+}