diff --git a/Tree/Tree.playground/Contents.swift b/Tree/Tree.playground/Contents.swift
index d59b7eedb..49cf41054 100644
--- a/Tree/Tree.playground/Contents.swift
+++ b/Tree/Tree.playground/Contents.swift
@@ -1,30 +1,9 @@
 //: Playground - noun: a place where people can play
 
-public class TreeNode<T> {
-  public var value: T
-
-  public weak var parent: TreeNode?
-  public var children = [TreeNode<T>]()
-
-  public init(value: T) {
-    self.value = value
-  }
-
-  public func addChild(_ node: TreeNode<T>) {
-    children.append(node)
-    node.parent = self
-  }
-}
-
-extension TreeNode: CustomStringConvertible {
-  public var description: String {
-    var s = "\(value)"
-    if !children.isEmpty {
-      s += " {" + children.map { $0.description }.joined(separator: ", ") + "}"
-    }
-    return s
-  }
-}
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4!")
+#endif
 
 let tree = TreeNode<String>(value: "beverages")
 
diff --git a/Tree/Tree.playground/Sources/Tree.swift b/Tree/Tree.playground/Sources/Tree.swift
new file mode 100644
index 000000000..fa49f25e9
--- /dev/null
+++ b/Tree/Tree.playground/Sources/Tree.swift
@@ -0,0 +1,39 @@
+public class TreeNode<T> {
+  public var value: T
+
+  public weak var parent: TreeNode?
+  public var children = [TreeNode<T>]()
+
+  public init(value: T) {
+    self.value = value
+  }
+
+  public func addChild(_ node: TreeNode<T>) {
+    children.append(node)
+    node.parent = self
+  }
+}
+
+extension TreeNode: CustomStringConvertible {
+  public var description: String {
+    var s = "\(value)"
+    if !children.isEmpty {
+      s += " {" + children.map { $0.description }.joined(separator: ", ") + "}"
+    }
+    return s
+  }
+}
+
+extension TreeNode where T: Equatable {
+  public func search(_ value: T) -> TreeNode? {
+    if value == self.value {
+      return self
+    }
+    for child in children {
+      if let found = child.search(value) {
+        return found
+      }
+    }
+    return nil
+  }
+}
diff --git a/Tree/Tree.playground/timeline.xctimeline b/Tree/Tree.playground/timeline.xctimeline
deleted file mode 100644
index bf468afec..000000000
--- a/Tree/Tree.playground/timeline.xctimeline
+++ /dev/null
@@ -1,6 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<Timeline
-   version = "3.0">
-   <TimelineItems>
-   </TimelineItems>
-</Timeline>
diff --git a/Tree/Tree.swift b/Tree/Tree.swift
index fa49f25e9..08cb61eb5 100644
--- a/Tree/Tree.swift
+++ b/Tree/Tree.swift
@@ -1,39 +1,40 @@
 public class TreeNode<T> {
-  public var value: T
+    public var value: T
 
-  public weak var parent: TreeNode?
-  public var children = [TreeNode<T>]()
+    public weak var parent: TreeNode?
+    public var children = [TreeNode<T>]()
 
-  public init(value: T) {
-    self.value = value
-  }
+    public init(value: T) {
+        self.value = value
+    }
 
-  public func addChild(_ node: TreeNode<T>) {
-    children.append(node)
-    node.parent = self
-  }
+    public func addChild(_ node: TreeNode<T>) {
+        children.append(node)
+        node.parent = self
+    }
 }
 
 extension TreeNode: CustomStringConvertible {
-  public var description: String {
-    var s = "\(value)"
-    if !children.isEmpty {
-      s += " {" + children.map { $0.description }.joined(separator: ", ") + "}"
+    public var description: String {
+        var s = "\(value)"
+        if !children.isEmpty {
+            s += " {" + children.map { $0.description }.joined(separator: ", ") + "}"
+        }
+        return s
     }
-    return s
-  }
 }
 
 extension TreeNode where T: Equatable {
-  public func search(_ value: T) -> TreeNode? {
-    if value == self.value {
-      return self
+    public func search(_ value: T) -> TreeNode? {
+        if value == self.value {
+            return self
+        }
+        for child in children {
+            if let found = child.search(value) {
+                return found
+            }
+        }
+        return nil
     }
-    for child in children {
-      if let found = child.search(value) {
-        return found
-      }
-    }
-    return nil
-  }
 }
+