 
 extension String {
     subscript (i: Int) -> Character {
-        return self[advance(self.startIndex, i)]
+        return self[self.startIndex.advancedBy(i)]
     }
     
     subscript (i: Int) -> String {

    }
    
    func maskedCardNumber(mask: String = "X") -> String {
        var eIndex = advance(cardNumber.endIndex, -4)
        var maskCharacters = ""
        
        for i in 0..<count(cardNumber.substringToIndex(eIndex)) {
            maskCharacters = maskCharacters + mask
        }
        

    }
    
    func formattedExpiration(divider: String) -> String {
        let idx = cardExpiration.startIndex.advancedBy(2)
        let month = cardExpiration.substringToIndex(idx)
        let year  = cardExpiration.substringFromIndex(idx)
        return "\(month)\(divider)\(year)"
    }
}
