1. If any cell is nonblank, demote it.
The only cells you should consider are those that are blank, so if a cell is
already taken, subtract 99 from its cell ranking.
2. Look for partially completed winning combinations.
Step through each winning combination. If any two cells have the same value but the third is blank, add a value to the third.
3. Find the highest cell.
After going through all the combinations, loop through the cell rankings
to see which is the highest cell ranking.