tarek wrote:BTW to atone for you absence maybe you should start by checking the posts from your famous thread ... Solving without pencilmarks

That old thing is still pinned here?

Thanks for the link, eleven! I'll check it out.

But now back to the topic. Did my corrected answer help OP?

The general rule is that all unavoidable sets that apply in a regular sudoku apply in a pencilmark sudoku as well, but some additional checks might be needed. An unavoidable set cannot be solved uniquely because cells outside the set cannot possibly provide the information to solve it. However, in a pencilmark sudoku it might already be partially solved from the inside by the puzzle generator, which has the power to remove whatever candidates it wants. Therefore we cannot rely on uniqueness techniques that only check if all candidates belong to some collection of candidates, we must also check that enough candidates are still present to make it an unavoidable set.

In this case the requirement is that when you focus only on the cells of the unavoidable set, every candidate that appears must appear at least twice in the row, column and box (within the set). If there is a candidate that appears in only one cell of a row within the set, it could eventually become a hidden single when you solve the rest of the puzzle. From this follows that the two columns here need to have pairwise identical sets of candidates. If a candidate appears in one cell, it has to appear in the other cell of the same row as well.

I believe this same check is all we need in pencilmark sudokus to validate just about all cases of simple unavoidable sets commonly used for solving. Have been trying to think of some counterexample, but can't come up with any now.