private void button_Click(object sender, RoutedEventArgs e)
        {
            Button button = sender as Button;
            int    digit  = -1;

            if (int.TryParse(button.Content.ToString(), out digit))
            {
                calc.AddDigit(digit);
            }
            else
            {
                switch (button.Tag)
                {
                case "Add":
                    calc.ProcessBinOp(CalculatorOperations.Add);
                    break;

                case "Sub":
                    calc.ProcessBinOp(CalculatorOperations.Sub);
                    break;

                case "Mul":
                    calc.ProcessBinOp(CalculatorOperations.Mul);
                    break;

                case "Div":
                    calc.ProcessBinOp(CalculatorOperations.Div);
                    break;

                case "Eq":
                    calc.ProcessBinOp(CalculatorOperations.Eq);
                    break;

                case "Invert":
                    calc.ProcessUnOp(CalculatorUnOperations.Invert);
                    break;

                case "Point":
                    calc.AddPoint();
                    break;

                case "Reciprocal":
                    calc.ProcessUnOp(CalculatorUnOperations.Reciprocal);
                    break;

                case "CE":
                    calc.Clear();
                    break;

                case "C":
                    calc.ClearAll();
                    break;

                case "Percent":
                    calc.ProcessBinOp(CalculatorOperations.Percent);
                    break;

                case "Sqr":
                    calc.ProcessUnOp(CalculatorUnOperations.Sqr);
                    break;

                case "Sqrt":
                    calc.ProcessUnOp(CalculatorUnOperations.Sqrt);
                    break;

                case "Backspace":
                    calc.ProcessUnOp(CalculatorUnOperations.Backspace);
                    break;
                }
            }
        }