BUG-Lite
BUG-Lite is a small-scale cousin of the BUG+1. The full Bivalue Universal Grave looks at the whole grid; BUG-Lite finds a partial pattern — a handful of cells whose two-candidate structure, if left as is, would let the puzzle finish two different ways. Because a fair Sudoku has one solution, that pattern must be broken, and breaking it hands you an elimination.
The idea
Look for a closed group of cells in which two digits alternate so that each of the digits appears exactly twice in every row, column and box the group touches. Such a group is a miniature "grave": you could swap the two digits all the way round it and satisfy every rule, giving a second solution. Since only one solution is allowed, at least one cell in the group must actually carry a different digit. Wherever a group cell has an extra candidate, the true solution has to use it there — so the two pattern digits can be removed from that cell, collapsing the grave.
A worked example
Six cells across three boxes each reduce to {4,6}, arranged so 4 and 6 each appear twice in every row, column and box they share — a swappable ring. One of those cells also holds a 1. If the puzzle is unique, that 1 cannot be idle: the solution must place it there. So 4 and 6 leave that cell, breaking the ring and usually leaving a single behind elsewhere.
A balanced {4,6} ring — each digit twice per row, column and box. Swapping would give a second solution, so the extra 1 (top-left) is forced, clearing 4 and 6 there and collapsing the grave.
Step by step
- Find a partial bivalue pattern where two digits each appear twice per unit it touches.
- Confirm the swap would give a valid second solution.
- Locate the extra candidate in a pattern cell.
- Eliminate the two pattern digits there to break the grave.
Common mistakes
- Not truly balanced. Each digit must appear exactly twice in every unit the pattern touches, or the swap is not free.
- Variant grids. Cages and diagonals can forbid the swap, so keep BUG-Lite to standard, uniquely-solved puzzles.
- No extra candidate. A perfectly balanced pattern with no extras signals a flawed puzzle, not an elimination.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.