@Test
public void testSplit_Leaf() {

    // split(0) = (0, 0)
    ID[] zeroSplit = zero.split();
    assertEquals(zero, zeroSplit[0]);
    assertEquals(zero, zeroSplit[1]);
    // split(1) = ((1,0), (0,1)
    ID[] oneSplit = one.split();
    assertEquals(oneZero, oneSplit[0]);
    assertEquals(zeroOne, oneSplit[1]);
}