@Test
    public void testDoubleNested(){
        String input = "((x * x) + (2 * y))";
        String var = "x";
        String expected = "(((x * 1) + (x * 1)) + ((2 * 0) + (y * 0)))";
        Differentiator diff = new Differentiator();
        assertEquals(expected, diff.evaluate(input, var));
    }