   /// `Mirror`.  In general, though, the observability of such
   /// mutations is unspecified.
   public init(reflecting subject: Any) {
-    if let customized? = subject as? CustomPlaygroundQuickLookable {
+    if let customized as CustomPlaygroundQuickLookable = subject {
       self = customized.customPlaygroundQuickLook()
     }
     else {

  /// A `String` argument selects the first `Child` with a matching label.
  /// An integer argument *n* select the *n*\ th `Child`.  For example::
  ///
  ///   var d = reflect(x).descendant(1, "two", 3)
  ///
  /// is equivalent to:
  ///
  /// .. parsed-literal::
  ///
  ///   var d = nil
  ///   let children = reflect(x).children
  ///   let p0 = advance(children.startIndex, **1**, children.endIndex)
  ///   if p0 != children.endIndex {
  ///     let grandChildren = reflect(children[p0].value).children

// without the ObjC Runtime.

/// Representation of the sub-structure and optional "display style"
/// of any arbitrary subject instance.
///
/// Describes the parts---such as stored properties, collection
/// elements, tuple elements, or the active enumeration case---that
/// make up a particular instance.  May also supply a "display style"
/// property that suggests how this structure might be rendered.
///
/// Mirrors are used by playgrounds and the debugger.
