Testing Equatable function between base class and its subclass but != seems not working as expected?

gistfile1.swift

// Creature

class Creature : Equatable {
}

func ==(lhs: Creature, rhs: Creature) -> Bool {
    return lhs === rhs
}

//	Monster

class Monster : Creature {
	let name: String
	init(name: String) {
		self.name = name
	}
}

func ==(lhs: Monster, rhs: Monster) -> Bool {
    return lhs.name == rhs.name
}

// (1)
//func !=(lhs: Monster, rhs: Monster) -> Bool {
//    return lhs.name != rhs.name
//}

let a = Creature()
let b = Creature()
let elmo1 = Monster(name: "Elmo")
let elmo2 = Monster(name: "Elmo")
let oscar = Monster(name: "Oscar")
let telly = Monster(name: "Telly")

a == b
a == a
a == oscar
a != oscar

elmo1 != elmo1
elmo1 != elmo2 // should be false without (1) ??
elmo1 == elmo1
elmo1 == elmo2

oscar == telly
oscar != telly