@Test
public void postOrder_graph_connectedRoots() {

  Walker<String> walker =
      DataType.GRAPH.newWalker(
          ImmutableListMultimap.of("foo", "bar", "bar", "dog", "zoo", "dog", "zoo", "cat"));
  assertThat(walker.postOrderFrom("foo", "zoo"))
      .containsExactly("cat", "zoo", "dog", "bar", "foo")
      .inOrder();
}