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

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

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