BUG
BUG stands for Bi-value Universal Grave. This is a configuration of the board, where all
remaining empty cells have exactly two candidates.
It can be shown that such a configuration implies that the puzzle does not have a unique solution.
Hence, if the creator of a Sudoku board guarantees that there is a unique solution, we can exploit this fact and avoid such a pattern.
The configuration which we are looking for is that all but one empty cells have exactly two candidates and
the remaining cell has exactly three candidates. By trying to eliminate (rather than place candidates) we will try to avoid a BUG pattern.
BUG - Example 1
Let us consider this example, where all cells have two candidates, except for A7, which has three.
Eliminating candidate 8, will do the same - leaving only one 8 in the same row, column and square.
However, eliminating candidate 5, leaves all rows, columns and squares with exactly two candidates per number (for example row A will contain exactly two 2s, two 5s and two 8s). This is a BUG and so the board will have more than one solution.
Hence eliminating the 5 in A7 leads to a contradiction and we should simply place it there.
Please note, that there are other methods, which depend on the uniqueness of a solution,
but we deliberately exclude them from our solver.
They all exploit an impossible pattern in a sudoku board and try to avoid such a configuration.
You can find more information on these methods on the SudokuWiki Webpage.
