Swift: function that returnss another function as its value

//swift

//demonstrating a function that returns another function as its value

func addALetter() -> (String -> String) {
    func addChars(char: String) -> String {
        var twoChars = ""
        var newChar = ""
        if char == "z" {
            newChar = "zy"
        } else {
        let alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y", "z"]
        let count = alphabet.count
        for i in 0..count {
            if char == alphabet[i] {
                twoChars = alphabet[i+1]
                newChar = char + twoChars
            }
        }

        }

        return newChar
    }
    return addChars
}

//let f = "g"
var s = addALetter()
s("c")