@Test
  public void shouldCorrectlyDiscoverIfElementIsInSET() {
    cut.insert(new Point2D(.1, .1));

    assertFalse(cut.contains(new Point2D(.0, .0)));
    assertTrue(cut.contains(new Point2D(.1, .1)));
  }
 @Test(expected = NullPointerException.class)
 public void shouldThrowNullPointerExceptionWhenCheckingIfSETContainsNull() {
   cut.contains(null);
 }