Nine: Sudoku That Explains

Every sudoku app has a hint button. You press it, a digit appears, and you have learned nothing. Next time you are stuck in the same place. Nine does it differently. You get one explanation per puzzle, and it fills in nothing. It names the technique, highlights the row, column or box where the proof is, and spells out the reasoning: "ONLY PLACE. In the bottom right box the 7 can only go here. The 7s in row 8 and column 9 rule out the other cells." "POINTING PAIR. In the bottom right box the 5 can only go in row 8. So the 5 is out everywhere else in row 8. That leaves column 2 with only one place for the 5: here." Then you place it yourself. You learn something instead of being handed the answer. THE PUZZLES Four difficulty levels, 30 levels each. Difficulty does not come from the number of given digits, but from the techniques needed to solve: from lone digits to triples and X-wing.…