@Test
public void testExpression() throws Exception {
Assert.assertEquals(new AddEvaluator(new DecimalEvaluator("1"), new DecimalEvaluator("2")),
parser("1+2").expression().e);
Assert.assertEquals(new SubtractEvaluator(new DecimalEvaluator("1"), new DecimalEvaluator("2")),
parser("1-2").expression().e);
Assert.assertEquals(new SubtractEvaluator(
new MultiplyEvaluator(new DecimalEvaluator("1"), new DecimalEvaluator("2")),
new DecimalEvaluator("2")),
parser("1*2-2").expression().e);
Assert.assertEquals(new AddEvaluator(