// leaf3D and gravity are subscribers of the leaf
assertEquals(2, treeLeaf.countSubscribers());
// leaf is subscriber of the leaf3D
assertEquals(1, treeLeaf3D.countSubscribers());
// assert the gravity is really one of the subscribers of the leaf
treeLeaf.deleteSubscriber(gravity);
assertEquals(1, treeLeaf.countSubscribers());
// assert the leaf3D is really one of the subscribers of the leaf