private Namespace(String[] levels) {
    this.levels = levels;
  }
@Test
  public void testNamespace() {
    String[] levels = {"a", "b", "c", "d"};
    Namespace namespace = Namespace.of(levels);
    assertThat(namespace).isNotNull();
    assertThat(namespace.levels()).hasSize(4);
    assertThat(namespace).hasToString("a.b.c.d");
    for (int i = 0; i < levels.length; i++) {
      assertThat(namespace.level(i)).isEqualTo(levels[i]);
    }
  }