@Test
 public void testRight() {
   assertEquals(new Coordinate(1, 0), Coordinate.ORIGIN.right());
 }
 @Test
 public void testDown() {
   assertEquals(new Coordinate(0, 1), Coordinate.ORIGIN.down());
 }
 @Test
 public void testLeft() {
   assertEquals(new Coordinate(-1, 0), Coordinate.ORIGIN.left());
 }
 @Test
 public void testUp() {
   assertEquals(new Coordinate(0, -1), Coordinate.ORIGIN.up());
 }