All the potential positions of the value {0} in the
{1} are in the same {2}.
Because the {1} must contain the value {0}, the {2} will have the
value {0} in one of the intersecting cells.
The other potential positions of the value {0} that are in the
{2} but not in the {1} are therefore not valid.
As a result, the cell {4} is the only remaining position of
the value {0} in its {1}.