val lcatSet: CodePointSet,
) {
  /**
   * Returns [input] in canonical form according to these rules, or null if no such canonical form
   * exists for it.
   */
  operator fun invoke(input: String): String? = invoke(Buffer().writeUtf8(input))

  internal operator fun invoke(input: BufferedSource): String? {
    // 1. Map.
    val mapResult = Buffer()
    while (!input.exhausted()) {
      val codePoint = input.readUtf8CodePoint()