func Char.Encrypt(code) {
    if !this.IsLetter() {
        return this
    }
    var offset = (this.IsUpper() ? 'A' : 'a').Order()
    return Char((this.Order() + code - offset) % 26 + offset)
}

func String.Encrypt(code) {
    var xs = []
    for c in this {
        xs.Add(c.Encrypt(code))
    }
    return String.Concat(values: xs)
}

func String.Decrypt(code) {
    this.Encrypt(26 - code);
}

var str = "Pack my box with five dozen liquor jugs."
print(str)
str = str.Encrypt(5)
print("Encrypted: \(str)")
str = str.Decrypt(5)
print("Decrypted: \(str)")