@Test public void testSum_NonLeafs() { assertEquals(one, oneZero.sum(zeroOne)); assertEquals(one, zeroOne.sum(oneZero)); ID expected = IDs.with(one, oneZero); assertEquals(expected, oneZero.sum(IDs.with(zero, oneZero))); }
abstract ID sum(ID other);
ID { abstract ID sum(ID other); }
ID { abstract ID sum(ID other);  }
ID { abstract ID sum(ID other);   }
ID { abstract ID sum(ID other);    }