* Returns the {@linkplain #isRegistrySuffix() registry suffix} portion of the domain name, or
   * {@code null} if no registry suffix is present.
   *
   * @since 23.3
   */
  public @Nullable InternetDomainName registrySuffix() {
    return hasRegistrySuffix() ? ancestor(registrySuffixIndex()) : null;
  }

  /**
   * Indicates whether this domain name ends in a {@linkplain #isRegistrySuffix() registry suffix},

    InternetDomainName domain = InternetDomainName.from("foo.city.yokohama.jp");
    assertTrue(domain.hasRegistrySuffix());
    assertThat(domain.registrySuffix().toString()).isEqualTo("yokohama.jp");

    // Behold the weirdness!
    assertFalse(domain.registrySuffix().isRegistrySuffix());
  }

  public void testRegistrySuffixMultipleUnders() {
    // PSL has both *.uk and *.sch.uk; the latter should win.

    InternetDomainName domain = InternetDomainName.from("foo.city.yokohama.jp");
    assertTrue(domain.hasRegistrySuffix());
    assertThat(domain.registrySuffix().toString()).isEqualTo("yokohama.jp");

    // Behold the weirdness!
    assertFalse(domain.registrySuffix().isRegistrySuffix());
  }

  public void testRegistrySuffixMultipleUnders() {
    // PSL has both *.uk and *.sch.uk; the latter should win.