In this new version I added "guess and validate".
In some difficult puzzles the solution can only be found by guessing a row or column at some point. After a guess, the puzzle can be solved further. At the end, the puzzle must be validated because a guess was made. If the puzzle is valid, the solution is found. Otherwise, another value for the row or column must be guessed.
Thank you so much for this comment!
I've been stuck on a puzzle for hours, trying to find the logical next move. After I entered it in your solver, it came up with "2 guess and validate" moves.
That explains a lot.
(it was the Logibrain binary #101 – very hard | 10x10)
Not sure I buy this argument. A puzzle where logic fails isn't really a "puzzle".