@Test
 public void test_add() {
   IMath math = new math.Math();
   assertEquals(5, math.add(2, 3));
   assertEquals(0, math.add(0, 0));
   assertEquals(0, math.add(-1, 1));
 }
 @Test
 public void test_mul() {
   IMath math = new math.Math();
   assertEquals(6, math.mul(6, 1));
   assertEquals(8, math.mul(4, 2));
   assertEquals(0, math.mul(5, 0));
 }
 @Test
 public void test_div() {
   IMath math = new math.Math();
   assertEquals(3, math.div(9, 3));
   assertEquals(5, math.div(15, 3));
   assertEquals(6, math.div(36, 6));
 }
 @Test
 public void test_sub() {
   IMath math = new math.Math();
   assertEquals(6, math.sub(6, 0));
   assertEquals(6, math.sub(9, 3));
   assertEquals(0, math.sub(-1, -1));
 }