cl-deck-builder2/doc/flowcharts/cluster.dot

// https://graphviz.org/Gallery/directed/cluster.html

digraph G {
    fontname="Helvetica,Arial,sans-serif"
    node [fontname="Helvetica,Arial,sans-serif"]
    edge [fontname="Helvetica,Arial,sans-serif"]

    subgraph cluster_0 {
	style=filled;
	color=lightgrey;
	node [style=filled,color=white];
	a0 -> a1 -> a2 -> a3;
	label = "process #1";
    }

    subgraph cluster_1 {
	node [style=filled];
	b0 -> b1 -> b2 -> b3;
	label = "process #2";
	color=blue
    }
    start -> a0;
    start -> b0;
    a1 -> b3;
    b2 -> a3;
    a3 -> a0;
    a3 -> end;
    b3 -> end;

    start [shape=Mdiamond];
    end [shape=Msquare];
}