lost type relationship - without Self

Ordered-withoutSelf.swift

/** 
* Example from: Protocol-oriented Programming in Swift - WWDC 2015 
* https://developer.apple.com/videos/play/wwdc2015/408/
*/
class Ordered {
  func precedes(other: Ordered) -> Bool { fatalError("implement me!") }
}

class Label : Ordered { var textL String = "" ... }

class Number: Ordered {
  var value: Double = 0
  override func precedes(other: Ordered) -> Bool {
    // forced down-cast here is generally a code smell 
    // even though we can make this an optional down-cast, it is still a sign that some type relationship is lost
    return value < (other as! Number).value
  }
}