So the JavaScript developers feel right at home...

js.swift

class NaNClass {}
let NaN = NaNClass()

@infix func == (n1: NaNClass, n2: NaNClass) -> Bool {
    return false
}

@infix func != (n1: NaNClass, n2: NaNClass) -> Bool {
    return true
}

@infix func + (a1: Array<Any>, a2: Array<Any>) -> String {
    return ""
}

NaN == NaN  // -> false
NaN != NaN  // -> true
[] + []     // -> ""

$ jsc
>>> NaN != NaN
true

now swift is ready to scale

web scale™

@kevin1 fixed!

import Foundation // Is there another way to get the C standard library?
let a = atan(4)
a == a // false
a === a // error, although to be fair it does the wrong thing in JS
a is a // meaningless in Swift, does the right thing in JS Harmony