several solving techniques which involve avoiding a deadly rectangle situation.
Deadly rectangles can only exist in invalid puzzles (they have
more than one solution).
The four cells shown on the left form a deadly rectangle.
There is no way to remove a candidate so there are two
to this invalid puzzle.
IMPORTANT: The four cells must reside in only two boxes.
The cells cannot reside in four different boxes.
(see this example)
There is a large number of uniqueness tests. Only a handful of
them are shown on this tutorial.
(Is this a valid solving technique?)