final class SimpleNode: Node {
    typealias Value = String
    var value: Value
    weak var parent: SimpleNode?
    var children: [SimpleNode]
    
    init(value: Value) {
        self.value = value
        self.children = []
    }
}

extension SimpleNode: Equatable, CustomStringConvertible {
    static func == (lhs: SimpleNode, rhs: SimpleNode) -> Bool {
        return lhs.value == rhs.value && lhs.parent == rhs.parent
    }

    public var description: String {
        return "\(value)"
    }
}

let root = SimpleNode(value: "Hand")

root.addChild(node: SimpleNode(value: "Thumb"))
root.addChild(node: SimpleNode(value: "Index finger"))
root.addChild(node: SimpleNode(value: "Middle finger"))
root.addChild(node: SimpleNode(value: "Ring finger"))
root.addChild(node: SimpleNode(value: "Little finger"))

print(root.lineBasedDescription)

/// Returns all parent nodes.
var ancestors: [Node]

/// Returns all parent nodes, including the current node.
var ancestorsIncludingSelf: [Node]

/// A Boolean value indicating whether the current node is the top node.
var isRoot: Bool

/// Returns the top node.
var root: Node

/// Adds a sub-node.
func addChild(node: Node)

/// Returns the number of children.
var degree: Int

/// Returns all descendants, traversing the entire tree.
var descendants: [Node]

/// A Boolean value indicating whether the node is without children.
var isLeaf: Bool

/// Returns all nodes with no children.
var leaves: [Node]

/// Returns the number of leaves.
var breadth: Int

/// A Boolean value indicating whether the node has children.
var isBranch: Bool

/// Returns all nodes with at least one child.
var branches: [Node]

/// Returns all other nodes with the same parent.
var siblings: [Node]

/// Returns all nodes (including the current node) with the same parent.
var siblingsIncludingSelf: [Node]

/// Returns the distance between a node and the root.
var depth: Int

/// The number of edges between the current node and the root.
var level: Int

var lineBasedDescription: String

let root = SimpleNode(value: "Apple")

let desktops = SimpleNode(value: "Desktops")
root.addChild(node: desktops)

let macPro = SimpleNode(value: "Mac Pro")
desktops.addChild(node: macPro)

let macMini = SimpleNode(value: "Mac Mini")
desktops.addChild(node: macMini)

let iMac = SimpleNode(value: "iMac")
desktops.addChild(node: iMac)

let notebooks = SimpleNode(value: "Notebooks")
root.addChild(node: notebooks)

let macBookPro = SimpleNode(value: "MacBook Pro")
notebooks.addChild(node: macBookPro)

let devices = SimpleNode(value: "Devices")
root.addChild(node: devices)

let handhelds = SimpleNode(value: "Handhelds")
devices.addChild(node: handhelds)

let ipod = SimpleNode(value: "iPod")
handhelds.addChild(node: ipod)

let iphone = SimpleNode(value: "iPhone")
handhelds.addChild(node: iphone)

let newton = SimpleNode(value: "Newton")
handhelds.addChild(node: newton)

let setTopBoxes = SimpleNode(value: "Set-top boxes")
devices.addChild(node: setTopBoxes)

let appleTV = SimpleNode(value: "Apple TV")
setTopBoxes.addChild(node: appleTV)


print(root.lineBasedDescription)