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

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

  ///     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 }
