   ///     for ch in chars {
   ///         print(ch, "-->", ch.uppercased())
   ///     }
+  ///     // Prints:
   ///     // e --> E
   ///     // é --> É
   ///     // и --> И

  ///     for ch in chars {
  ///         print(ch, "-->", ch.wholeNumberValue)
  ///     }
  ///     // 4 --> 4
  ///     // ④ --> 4
  ///     // 万 --> 10000
  ///     // a --> nil
  public var wholeNumberValue: Int? {
    guard _isSingleScalar else { return nil }

  ///     for ch in chars {
  ///         print(ch, "-->", ch.hexDigitValue)
  ///     }
  ///     // Prints:
  ///     // 1 --> Optional(1)
  ///     // a --> Optional(10)
  ///     // Ｆ --> Optional(15)
  ///     // g --> nil
  public var hexDigitValue: Int? {
    guard _isSingleScalar else { return nil }
