<graph>
  <node id="a" />
  <node id="b" />
  <node id="c" />
  <node id="d" />
  <node id="e" />
  <edge from="a" to="b" />
  <edge from="a" to="c" />
  <edge from="b" to="c" />
  <edge from="b" to="d" />
  <edge from="c" to="d" />
  <edge from="c" to="e" />
  <edge from="e" to="e" />
</graph>