@Test
    public void simpleMulTest() {
        String input = "(x * 2.0)";
        Lexer lexer = new Lexer(input);
        Parser parser = new Parser(lexer);
        Expression exp = parser.parse();
        Token var = new Token("x");
        Token num = new Token("2.0");
        Variable expectVar = new Variable(var);
        Number expectNum = new Number(num);
        Multiplication expected = new Multiplication(expectVar, expectNum);
        assertEquals(exp.toString(), expected.toString());
    }