rx

        Observable.from(commands)
            .merge()
            .scan(CalculatorState.CLEAR_STATE) { a, x in
                return a.tranformState(x)
            }
            .debug("debugging")
            .subscribe(onNext: { [weak self] calState in
                self?.resultLabel.text = calState.inScreen
                switch calState.action {
                case .operation(let operation):
                    switch operation {
                    case .addition:
                        self?.lastSignLabel.text = "+"
                    case .subtraction:
                        self?.lastSignLabel.text = "-"
                    case .multiplication:
                        self?.lastSignLabel.text = "x"
                    case .division:
                        self?.lastSignLabel.text = "/"
                    }
                default:
                    self?.lastSignLabel.text = ""
                }
            })
            .addDisposableTo(disposeBag)