digraph undirected {
	edge [ arrowhead="none" ];
	node [ shape="circle" ];
	{
		rank=same;
		2;
		4;
	}
	{
		rank=same;
		3;
	}
	{
		rank=same;
		1;
		5;
	}
	1 -> 2;
	3 -> 2;
	3 -> 4;
	2 -> 4;
	4 -> 5;
	5 -> 6;
}