Two 1s a step apart share the three green cells. Imagine two worlds. In the world with a ghost in green, both 1s are satisfied at once and the rest of both windows is ×. In the world without, each window keeps its own ghost. Write down only what holds in both worlds. That is case analysis — logic calls it disjunction elimination.

In the app, one new tip arrives for each day you play.

▶ Try 50 rounds in your browser A free demo, no install. All 2,000 rounds are in the app.