@Override
  public String toString() {
    return "{" +
        (flags.stream()
            .map(Enum::name)
            .collect(Collectors.joining(", ")))
        + "}";
  }
@Test
  public void testToString() throws Throwable {
    // empty
    assertStringValue("{}");
    assertConfigurationStringMatches("");

    // single value
    flagSet.enable(SimpleEnum.a);
    assertStringValue("{a}");
    assertConfigurationStringMatches("a");

    // add a second value.
    flagSet.enable(SimpleEnum.b);
    assertStringValue("{a, b}");
  }