Bug

Index
Bug
When every unsolved cell in puzzle has two possible candidates a Bivalue Universal Grave (BUG) position is establised. In such a pattern, in every group there are exactly two of every candidate.

When the puzzle is in a BUG position, it has either zero or more than one solution. If you know if a puzzle has only one solution, any move which turns a grid into a BUG position is invalid. The next move should be the one that does not create a BUG position.

In the example below, the yellow highlighted cell must be an 8, because any other move create the BUG position. The general rule is that when all cells but one have two candidate values, the candidate value in the cell with three values that occurs more than two times in a group can be excluded.
Bug