@Test
 public void multiplyIntegers() {
   Number actual = evaluate(a._mul(b), intPoint(6, 7));
   assertThat(actual.intValue(), equalTo(42));
 }
 @Test
 public void integerPolynomial() {
   // a**3 + a*b + 7
   Number actual = evaluate(a._mul(a)._mul(a)._plus(a._mul(b))._plus(7), intPoint(3, -8));
   assertThat(actual.intValue(), equalTo(10));
 }