gravity.addFallingLeaf(treeLeaf);
assertEquals(3, gravity.getFallingLeaves().size());
assertEquals(3, gravity.getGravity3D().getGroup().numChildren());
// gravity is subscriber of the leaf
assertEquals(1, treeLeaf.countSubscribers());
treeLeaf.deleteSubscriber(gravity);
// assert the gravity is really one of the subscribers of the leaf
assertEquals(0, treeLeaf.countSubscribers());
}
public void testUpdate() {