val codePoint = validateBuffer.readUtf8CodePoint()
      if (codePoint in prohibitSet) return null

      if (first) {
        firstIsRandalcat = codePoint in randalcatSet
      } else if (firstIsRandalcat) {
        // 'If a string contains any RandALCat character, the string MUST NOT contain any LCat
        // character.'
        if (codePoint in lcatSet) return null