compartment happened to be 'empty' too, the Square would be 'empty': and, if it happened to be 'occupied', the Square would be 'occupied'. So, as we do not know WHICH is the case, we can say nothing about THIS Square.
The other Square, the xy'-Square, we know (as in the previous example) to be 'occupied'.
If, then, we transfer our marks to the smaller Diagram, we get merely this:--
-----------
| | |
| | 1 |
| | |
-----------