graph ex3pt1 {
	node [texmode="math",shape="none"];
	S0 [label="S"];
	S1 [label="S"];
	a0 [label="a"];
	S2 [label="S"];
	S3 [label="S"];
	a1 [label="a"];
	S4 [label="S"];
	b0 [label="b"];
	b1 [label="b"];
	b2 [label="b"];
	S0 -- S1;
	S0 -- a0;
	S0 -- S2;
	S1 -- S3;
	S1 -- a1;
	S1 -- S4;
	S2 -- b0;
	S3 -- b1;
	S4 -- b2;
}