Sudokup

Ordinary sudoku, with one new rule: fill the bottom band and the board climbs. Those three rows leave and a fresh band arrives above, already consistent with what is left. Nothing moves until you move it.

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How it plays

• It is sudoku. Every row, every column and every three-by-three box holds one to nine, once each. If you have done one before you have done this one.
• Complete the bottom band to advance. Three rows, three boxes, twenty-seven cells — fill them and the board climbs a band. That is the whole twist.
• You reach up to solve down. The bottom band's column clues run against the two bands above it, so the information you need is in the part of the board you know least about. Work ahead and nothing is wasted: those bands are your next two targets.
• Press a cell and drag to a value. The three small digits in an empty cell are the only values its column can still take — never more than three, which is why there is no keypad to reach for.
• No erase, no undo. There is nothing to take back, because a wrong value is refused rather than written.

Why there are only ever three candidates

• Every three consecutive bands form a whole sudoku. Subtract one window's columns from the next and the arithmetic says a column's three values must repeat every third band — so the band arriving at the top holds exactly the values of the band you have just finished.
• That is free information, so the game gives it to you rather than rewarding whoever remembered. It is why a cell shows three ghosted digits and why picking one is a flick rather than a hunt.
• Which of the three is right is still yours: the ghosts are the column's, and row and box are the puzzle.

Never a forced guess

• Every band is checked before you see it: a solver limited to the techniques a person actually uses has to be able to finish it from the clues alone, or a clue goes back.
• It is checked from the givens only — not from whatever you happened to work out first — so the promise holds whatever order you solve in.
• And you cannot ruin a strip by trying: a value that is not the answer is refused rather than written, so the board only ever moves toward being finished.
• Scores are checked by replaying your moves on the server, so a leaderboard is what happened.