Swift by example class extensions
When you add an extension to an existing type,
the extension changes are available on existing
instances of that type, even if they were declared
before the extension. 

var someInt = 3

extension Int {
    func isOdd () -> Bool{
        return self % 2 != 0
    }

    func isEven() -> Bool {
        return !isOdd()
    }

    func times(task: (Int)->()) {
        for i in 0..<self {
            task(i)
        }
    }
}

print(someInt.isOdd())                 // true
print(2.isEven())                      // true

2.times({(Int i) in
    print(i)                           // 0, 1
})