assertTrue(domain.hasRegistrySuffix());
    assertEquals("essex.sch.uk", domain.registrySuffix().toString());
    assertEquals("www.essex.sch.uk", domain.topDomainUnderRegistrySuffix().toString());
  }

  public void testEquality() {
    new EqualsTester()
        .addEqualityGroup(idn("google.com"), idn("google.com"), idn("GOOGLE.COM"))

   *
   * @throws IllegalStateException if this domain does not end with a registry suffix
   * @since 23.3
   */
  public InternetDomainName topDomainUnderRegistrySuffix() {
    if (isTopDomainUnderRegistrySuffix()) {
      return this;
    }
    checkState(isUnderRegistrySuffix(), "Not under a registry suffix: %s", name);
    return ancestor(registrySuffixIndex() - 1);