drwho -- what's the difference between nishio and guessing?
There is not much difference between nishio and guessing. I would call it a guess if you actually work out the puzzle until you hit a dead end. I would call it nishio if you work out the puzzle in your head to a dead end point. Using the green markup digits is not necessarily the same as actually working out the puzzle. although it can be if you only fill in one green digit per empty cell. If you are using the green digits in the conventional way to show all the possible candidates in each empty cell then I would argue that you are working out the puzzle in your head with just a little memory aid from the green digits.
With the 2 color (red and green) markup system available here, you can implement guessing in a rather sophisticated way. You can fill in all the cells with green digits indicating what is possible in each empty cell. Then you can turn one of the green digits red in each cell and see if that leads to a successful solution. If not you can turn all the red digit back to green and try a different guess using the red digits to indicate your guess.
Generally, the nishio technique is looking for a fairly short chain of inference that leads to an impossibility. Guessing may require going right down to the last several cells before you realize that you have hit a dead end.
Difficulty score 68. No guessing, no nishio, just good old alternate inference chains.
I just call on my guardian angle
done, with greens
Did it, no green, two guesses, one unlucky, one lucky. As usual, the more productive looking guess was the wrong guess.