MUST go into No. 13.

The final result is


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



And now how much of this information can usefully be transferred
to the smaller Diagram?

Let us take its four compartments, one by one.

As to No. 5? This, we see, is wholly 'empty'. (So mark it with a
grey counter.)

As to No. 6? This, we see, is 'occupied'. (So mark it with a red
counter.)

As to No. 7? Ditto, ditto.