}

  @Test fun prohibitionPrivateUse() {
    assertThat(stringPrep("\uf8ff")).isNull()
    assertThat(stringPrep("\udbff\udffd")).isNull() // Note that this is one code point.
  }

  @Test fun prohibitionNonCharacter() {
    assertThat(stringPrep("\ufdd0")).isNull()
    assertThat(stringPrep("\ufffe")).isNull()
    assertThat(stringPrep("\udbff\udffe")).isNull()
  }

  /**