// initial join should replace Vector0
v=v.join(AxisVector.create(1, 3));
assertEquals(AxisVector.class,v.getClass());
// second join should create JoinedVector
v=v.join(Vector.of(1,2,3));
assertEquals(JoinedVector.class,v.getClass());
// third join should promote to JoinedMultiVector
v=v.join(ZeroVector.create(3));
assertEquals(JoinedMultiVector.class,v.getClass());