public static Index simple(String name) {
    return new Index(name, false);
  }
@Test
  public void simple_index_name_can_not_contain_underscore_except__all_keyword() {
    // doesn't fail
    Index.simple("_all");

    assertThatThrownBy(() -> Index.simple("_"))
      .isInstanceOf(IllegalArgumentException.class)
      .hasMessage("Index name must be lower-case letters or '_all': _");
  }