* surrogates with '?', we can't test this behavior with Java Strings. Instead, pass the surrogate
   * code itself encoded as UTF-8 sequence.
   */
  @Test fun prohibitionSurrogateCodes() {
    // UTF-8 encoding of the high surrogate U+D800.
    assertThat(stringPrep(Buffer().write("eda080".decodeHex()))).isNull()
  }

  @Test fun prohibitionInappropriateForPlainText() {