The what-if technique says: suppose a square has a bulb, or doesn’t, and follow what that forces. If a rule breaks, it’s the other way. It’s the technique for when nothing simpler works anywhere.
This is one of the Light Up techniques. Each page shows its technique on the same 12×12 from the game, Hard level 4.
The rule
- Check that no simpler technique finds anything.
- Pick a square that’s still open, and suppose it has a bulb (or doesn’t).
- Follow what that forces with the simple techniques: light, numbers, dark squares.
- If a number can’t be met, or a square can’t be lit, it’s the other way.
In a real puzzle
On the 12×12, it’s first needed at step 26, straight after the number corner. The 1 in row 4, column 2 needs its bulb either to its left, in row 4, column 1, or below it. Suppose it goes to the left. That bulb satisfies the 1, so the square below the 1 is ruled out. Two steps on, the 1 in row 6, column 2 has no bulb and no square left for one. So row 4, column 1 has no bulb, and at the next step the 1’s bulb goes below it.
How often you’ll need it
Never on Easy or Medium, and on every Hard level, about 7 times each on average (6.9). On the 12×12 here it’s 1 of the 42 steps.