public CalculatorViewModel(IEquationParser equationParser, Calculator calculator)
        {
            _equationParser = equationParser;
            _calculator = calculator;

            ExecuteCommand = new DelegateCommand(Execute);
        }
        public void WhenExecutingAndAddEquation_ThenResultShouldEqualThree()
        {
            // Arrange
            var calculator = new Calculator();
            var equation = new Equation()
                {
                    Tokens = new List<Token>()
                        {
                            new Token() {Value = 1m, Type = TokenType.Value},
                            new Token() {Value = "+", Type = TokenType.Operator},
                            new Token() {Value = 2m, Type = TokenType.Value},
                        }
                };

            // Act
            var result = calculator.Solve(equation);

            // Assert
            Assert.AreEqual(3m, result);
        }