Mental Royale
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BUG+1

Advanced Logic32 / 32

One cell saves the whole board from ambiguity.

BUG stands for Bivalue Universal Grave: a board where every unsolved cell holds exactly two candidates and every candidate appears exactly twice per unit. Such a board can't have a unique solution — every solution has a mirror twin.

BUG+1 is the moment just before the grave: all cells are bivalue except a single one with three candidates. Whichever of its digits appears three times in its row, column and box is the escape hatch — it must be placed, or the board falls into the BUG.

See it in action

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Step 1 of 4

The setup

Late in a puzzle, when nearly every cell shows a clean two candidates, look for the lone three-candidate cell. Count its digits in the cell's units: the one appearing three times is the answer. Step through this real position to see it happen.

Practice

Find the lone three-candidate cell and place the digit that defuses the BUG.

Drill 1 of 29

Tap the cell, then the digit — or type it.