-------------------
| 1 | 0 |
| _____|_____ |
| | | | |
| | | 0 | |
| | | | |
-------------------


Once more: how shall we interpret this, with regard to x and y?


-------------------
| 0 | 1 |
| _____|_____ |
| | | | |
| | | | |
| | | | |
-------------------


This tells us, as to the xy-Square, that ONE of its compartments
is 'empty'. But this information is quite useless, as there is no
mark in the OTHER compartment. If the other 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