   func mutate(action: Action) -> Observable<Mutation> {
     switch action {
     case .increase:
-      return Observable.just(Mutation.increaseValue)
+      return Observable.just(Mutation.increaseValue) // Action.increase -> Mutation.increaseValue
     case .decrease:
-      return Observable.just(Mutation.decreaseValue)
+      return Observable.just(Mutation.decreaseValue) // Action.decrease -> Mutation.decreaseValue
     }
   }
 


  override func viewDidLoad() {
    super.viewDidLoad()
    reactor = CounterViewReactor() // this makes `bind(reactor:)` gets called
  }

  // Called when the new value is assigned to `self.reactor`

import RxCocoa
import RxSwift

// Conform to the protocol `View` then the property `self.reactor` will be available.
final class CounterViewController: UIViewController, View {
  @IBOutlet var decreaseButton: UIButton!
  @IBOutlet var increaseButton: UIButton!
