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setOfNumbers.isEmpty  // returns true or false depending in the set.

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setOfNumbers.remove(6)     // removes an element
setOfNumbers.removeAll() // removeAll() remove all elements

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setOfNumbers.count

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setOfNumbers.insert(6)

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var setOfNumbers = Set<Int>() // you can change 'Int' with hashable type you want to work with.

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class ChooseViewController: UIViewController {
    
    @IBOutlet weak var sunButton: SpaceCustomButton!
    @IBOutlet weak var mercuryButton: SpaceCustomButton!
    @IBOutlet weak var venusButton: SpaceCustomButton!
    @IBOutlet weak var earthButton: SpaceCustomButton!
    @IBOutlet weak var moonButton: SpaceCustomButton!
    @IBOutlet weak var marsButton: SpaceCustomButton!
    @IBOutlet weak var jupiterButton: SpaceCustomButton!
    @IBOutlet weak var saturnButton: SpaceCustomButton!

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@IBAction func changeButtonDidTapped(_ sender: Any) {
        let chooseVC = storyboard?.instantiateViewController(identifier: "chooseVC") as! ChooseViewController
        chooseVC.modalPresentationStyle = .fullScreen
        chooseVC.delegate = self // This line of code
        self.present(chooseVC, animated: true, completion: nil)
}

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 var delegate: ChoosePlanetMoonDelegate!

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class MainViewController: UIViewController, ChoosePlanetMoonDelegate {
    @IBOutlet weak var mainPlanetImage: UIImageView!
    @IBOutlet weak var changeButton: UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    @IBAction func changeButtonDidTapped(_ sender: Any) {
        let chooseVC = storyboard?.instantiateViewController(identifier: "chooseVC") as! ChooseViewController

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import UIKit

protocol ChoosePlanetMoonDelegate {
    func didChoosePlanetWith(image: UIImage)
}