@Test
public void breadthFirst_disjointRoots() {

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