public void invalidNumber() {
   assertFalse(CheckSum.of(ACCOUNT_NUMBER_3).isValid());
   assertFalse(CheckSum.of(ACCOUNT_NUMBER_4).isValid());
 }
 public void validNumber() {
   assertTrue(CheckSum.of(ACCOUNT_NUMBER_1).isValid());
   assertTrue(CheckSum.of(ACCOUNT_NUMBER_2).isValid());
 }