All the potential positions of the value {1} in {2} different
{3}s are in the same {2} {4}s. Because the number of
{3}s matches the number of {4}s, and because each
{3} must contain the value {1}; each
{4} will have a {1} in one of the intersecting cells.
The other potential positions of the value {1} that are in the
{4}s but not in one of the {3}s can therefore be removed.