private static final ImmutableSet<String> PS =
      ImmutableSet.<String>builder().addAll(RS).addAll(PS_NOT_RS).build();

  private static final ImmutableSet<String> NO_PS =
      ImmutableSet.of("www", "foo.ihopethiswillneverbeapublicsuffix", "x.y.z");

  /**
   * Having a public suffix is equivalent to having a registry suffix, because all registry suffixes
   * are public suffixes, and all public suffixes have registry suffixes.
   */