A BUG occurs when a sudoku has, in all rows, columns, and boxes, each candidate appearing twice in each UNIT. This causes the Sudoku to have two solutions or no solution.
In a Universal Bivalve Grave, all cells in which no solution number has been inserted contain exactly two candidates. The only exception is the cell that contains three candidates.
For the existence of a BUG, it is necessary that there are, in all cells of the Sudoku, bi-value cells, with the exception of one.
In the Sudoku below, in row D, column 8, and box 6, there are two pairs of candidate numbers, except for the number “1” that appears three times. The green cell sees all three units. The number “1” is the solution for this cell D8.
This is called BUG+1, as there is only one extra number in the target cell.
Be careful. If in the above Sudoku, the green cell had exactly two candidates per UNIT, there would be no number to choose, and for sure, some other UNIT would have more than two candidate numbers.
BUG +2
In the example below there is the BUG +2 condition, where two cells have three candidates, the number “8” is a solution in both cells.
