
BUG stands
for Bivalue Universal Grave. It is similar to a deady rectangle in that it
involves avoiding a multiple solution situation.
A BUG exists when all remaining cells:
 are bivalue
 all candidates exist twice in their respective houses
When this occurs, there is more than one solution so the idea is to
avoid a BUG situation.
This technque is also known as BUG+1 indicating that one cell has an
extra candidate. This becomes the target cell.
The cell in row 4 col 5 has three candidates (4 8 9). Of those
candidates, each is present twice in each house except for the 9 which
is present three times in row 4, col 5 and box 5. If the 9 in row
4 col 5 were eliminated, the BUG would be complete and the puzzle would
be invalid.
Therefore the target cell must be a 9.

