Hidden Unique Rectangle

Hidden Unique Rectangle identifies a deadly pattern where the UR digit is 'hidden' in the roof cells' shared house. If the digit appears only in the roof cells within that house, it can be eliminated to prevent the deadly pattern.

Technique

1. Find a potential deadly rectangle with floor cells containing only {X,Y}.
2. Identify the roof cells' shared house (row, column, or block).
3. Check if one UR digit appears in that house only at the roof cells.
4. That digit is 'hidden' - eliminate it from the roof cells to prevent the deadly pattern.

Tips

• Hidden URs are harder to spot than standard URs because the key digit is hidden among other candidates.
• Focus on the roof cells' shared house and check for digits confined to just those cells.
• The technique relies on uniqueness assumption.