@Test
public void breadthFirst_tree_connectedRoots() {

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