[Request] Checkpoint feature

[kingjon3377]

In "Hard" and "Challenge" difficulty puzzles, I often reach a point where (as far as I can tell) I can make no more progress by ordinary deduction ("this cell must be N because..." or "this cell cannot be N because..."), so I have to start guessing, playing deductively from there, and seeing if I reach a contradiction, then when I do undoing to the point where I guessed (and hope I correctly remember which cell it was)

After a number of games like this, it would be really nice to have a way to save a "checkpoint" before making such a speculative guess, and then press a single button (possibly with a confirmation prompt, like when resetting the board) to jump back to that point.