Treating instances of a protocol type as a different subclass

Now, we will take advantage of the possibility that Swift offers us to extend an existing class to add specific members. In this case, we will add an instance method to the previously defined AngryCat class. The following lines add the doSomethingWithAnAngryCat method to the existing AngryCat class:

public extension AngryCat {
    public func doSomethingWithAnAngryCat(cat: AngryCat) {
        if let angryCatAlien = cat as? AngryCatAlien {
            angryCatAlien.appear()
        } else if let angryCatKnight = cat as? AngryCatKnight {
            angryCatKnight.unsheathSword()
        } else if let angryCatWizard = cat as? AngryCatWizard {
            print("My spell power is \(angryCatWizard.spellPower)")
        } else {
            print("This AngryCat doesn't have cool skills.")
        }
    }
}

The doSomethingWithAnAngryCat method receives an AngryCat instance (cat) and uses the conditional type casting operator (as?) to...