@Test
    public void simpleVarTest() {
        String input = "(x)";
        Lexer lexer = new Lexer(input);
        Parser_v3 Parser_v3 = new Parser_v3(lexer);
        Expression exp = Parser_v3.parse();
        Token var = new Token("x");
        Variable expected = new Variable(var);
        assertEquals(exp.toString(), expected.toString());
    }