How to have multiple definitions for a function in swift

Question

I am following a stanford online lecture series in which the professor demoes a calculator app. I am posting the viewcontroller.swift file code below and a screen shot of my current main.storyboard.

What I need to know, is how am I supposed to define multiple definitions for the same function? as you can see, I need to define a function operation which may have one or two variables as inout. I looked through the swift reference on why I am getting this error here. However, it does not mention anything close to what he does. I believe this is due to some changes in swift version, although I'm not sure.

I went through the reference and found the declaration in the format func funcName ( Double... ) to pass variable number of parameters, but I don't exactly know how I am to use that kind of instantiation here.

ViewController.swift:

import UIKit

class ViewController: UIViewController {

@IBOutlet var display: UILabel!

var userIsInTheMiddleOfTyping = false

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

@IBAction func numberPressed(sender: UIButton) {
    // We need to get the value from the current label
    // And then we need to update it with appending the pressed digit at the ending.
    let digit = sender.currentTitle!
    if ( userIsInTheMiddleOfTyping ){
        display.text = display.text! + digit
    } else{
        display.text = digit
        userIsInTheMiddleOfTyping = true
    }
}

@IBAction func operate(sender: UIButton) {
    let operation = sender.currentTitle!
    if userIsInTheMiddleOfTyping{
        enter()
    }
    switch operation {
    case "*": performOperation { $0 * $1 }
    case "/": performOperation { $1 / $0 }
    case "+": performOperation { $0 + $1 }
    case "-": performOperation { $1 - $0 }
    case "sqrt": performOperation( sqrt($0))
    default: break
    }
}

func performOperation( operation: (Double,Double)->Double){
    if operandStack.count >= 2{
        displayValue = operation(operandStack.removeLast() , operandStack.removeLast())
        enter()
    }
}

func performOperation( operation: Double -> Double ){
    if operandStack.count >= 1{
        displayValue = operation(operandStack.removeLast())
        enter()
    }
}

var operandStack = Array<Double>()

@IBAction func enter() {
    userIsInTheMiddleOfTyping = false
    operandStack.append(displayValue)
    print(operandStack)
}
var displayValue: Double{
    get{
        return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
    }
    set{
        display.text = "\(newValue)"
        userIsInTheMiddleOfTyping = false
    }
}

}

Main.StoryBoard: