Value of tuple Void (aka ()) has no member "isSuperset"

I created a union between two character sets to be able to use a period with the decimalDigits character set.

 func textField(_ textField: UITextField, shouldChangeCharactersIn     range: NSRange, replacementString string: String) -> Bool {

            let allowed = CharacterSet.decimalDigits
            let period = CharacterSet.init(charactersIn: ".")
            let both = CFCharacterSetUnion(allowed as! CFMutableCharacterSet, period as CFCharacterSet)

            let characterSet = NSCharacterSet(charactersIn: string)

            return both.isSuperset(of: characterSet as CharacterSet)
        }

however, returning "both.isSuperset(of: characterSet as CharacterSet)". how to correct?

Update 1: Errors displayed with the current suggested answer

Update 2: latest update

Try doing this instead:

func textField(_ textField: UITextField, shouldChangeCharactersIn     range: NSRange, replacementString string: String) -> Bool {

    var allowed = CharacterSet.decimalDigits
    let period = CharacterSet.init(charactersIn: ".")
    allowed.formUnion(period)

    //UNCOMMENT when isSuperset is working
    //let characterSet = CharacterSet(charactersIn: string)
    //return allowed.isSuperset(of: characterSet)

    // Swift 3 appropriate solution 
    let isSuperset = string.rangeOfCharacter(from: allowed.inverted) == nil
    return isSuperset
}

the basis for which I found here.

Even better, make "allowed" (or "both", whatever you decide to name it) a property that gets created in viewDidLoad so you don't recreate and union the character sets each time a character is typed.