Arie, can you please explain what is the technical difference between these two cases:

```python
class A:
    __slots__ = ('a', )
    # fields

class B(A):
    __slots__ = ('a', 'b')
    # fields
```

And:

```python
class C:
    __slots__ = ('a', )
    # fields

class D(C):
    __slots__ = ('b', )
    # fields
```

?