X-Loop (X-Cycle)
In this strategy, we build loops by alternately chaining weak links and strong links
- A strong link means that if a candidate is OFF in the first cell, it must be ON in the second cell, and vice versa. Weak links are less certain: if a candidate is ON in one cell, the other must be OFF, but if it is OFF in the first cell, the second cell could still be ON or OFF (if there are more than two candidates in the unit)
- Consider these cells. This chain forms a continuous loop of candidate 5
- At each end of any link, there must be a candidate 5 in one of the two cells. This allows us to safely eliminate 5 from any cell that can see both of these cells at the same time
