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