This is the final extension of the
X-wing idea. In each of the rows 1, 2, 4 and 7, a 7 must appear as
the solution in one of the highlighted cells.
Rotating your thinking through 90 degrees will lead to the conclusion that in columns 1, 2, 5 and 6, the only
cells which can have 7 as a solution are those which are highlighted. Any 7s elsewhere in columns 1, 2, 5 and 6
cannot be part of the solution, and may be removed. There are four such examples, and one of them (C2-R9)
results in a
Single Candidate situation.