@Test
public void breadthFirst_breadthFirst() {

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