digraph stack {
	ranksep=1;

	subgraph cluster1 {
		color=none
		sheet [width=4; shape=rect]
	}

	subgraph cluster2 {
		color=none
		label="composites"
		labeljust="r";
		bgcolor="#BBBBBB";

		group        [shape=rect]
		group_ref    [shape=rect]
		
		standalone   [shape=point; label=""; width=0]
	}

	subgraph cluster3 {
		color=none
		label="atoms"
		labeljust="r";
		bgcolor="#BBBBBB";

		line
		spline
		arc
		polygon
		text
		connection
	}

	sheet -> group
	sheet -> group_ref
	sheet -> standalone [arrowhead=none]

	group -> group
	group -> group_ref [constraint = false]

	group -> line
	group -> spline
	group -> arc
	group -> polygon
	group -> text
	group -> connection

	group_ref -> line [style=invis]


	standalone -> line         [constraint = false]
	standalone -> spline       [constraint = false]
	standalone -> arc          [constraint = false]
	standalone -> polygon      [constraint = false]
	standalone -> text         [constraint = false]
	standalone -> connection   [constraint = false]
}
