John has started a sudoku puzzle and shows me his completed work, asking whether he's made any errors. There are no obvious errors (a number repeated in a row or subgrid). I know of two ways to determine whether he's erred:

1. Continue solving the puzzle (without error) where John has left off, and see whether I arrive at an impossible state. If so, John erred. If not, I'll complete the puzzle, and John hasn't erred.
2. Start the puzzle over again and solve it (without error) until either (a) I have filled in every square John has, and with the same numbers, in which case John hasn't erred; or (b) I have filled in some square John has, but with a different number, in which case John has erred.

Is there another way to determine whether John has erred? Specifically, is there a way that doesn't involve redoing or continuing the solution, but, rather, involves merely examining what John has done?

John has started a sudoku puzzle and shows me his completed work, asking whether he's made any errors. There are no obvious errors (a number repeated in a row or subgrid). I know of two ways to determine whether he's erred:

1. Continue solving the puzzle (without error) where John has left off, and see whether I arrive at an impossible state. If so, John erred. If not, I'll complete the puzzle, and John hasn't erred.
2. Start the puzzle over again and solve it (without error) until either (a) I have filled in every square John has, and with the same numbers, in which case John hasn't erred; or (b) I have filled in some square John has, but with a different number, in which case John has erred (assuming that the puzzle has a unique solution).

Is there another way to determine whether John has erred? Specifically, is there a way that doesn't involve redoing or continuing the solution, but, rather, involves merely examining what John has done?

John has started a sudoku puzzle and shows me his completed work, asking whether he's made any errors. There are no obvious errors (a number repeated in a row or subgrid). I know of two ways to determine whether he's erred:

1. Continue solving the puzzle (without error) where John has left off, and see whether I arrive at an impossible state. If so, John erred. If not, I'll complete the puzzle, and John hasn't erred.
2. Start the puzzle over again and solve it (without error) until either (a) I have filled in every square John has, and with the same numbers, in which case John hasn't erred; or (b) I have filled in some square John has, but with a different number, in which case John has erred.

Is there another way to determine whether John has erred? Specifically, is there a way that doesn't involve redoing or continuing the solution, but involves merely examining what John has done?

John has started a sudoku puzzle and shows me his completed work, asking whether he's made any errors. There are no obvious errors (a number repeated in a row or subgrid). I know of two ways to determine whether he's erred:

1. Continue solving the puzzle (without error) where John has left off, and see whether I arrive at an impossible state. If so, John erred. If not, I'll complete the puzzle, and John hasn't erred.
2. Start the puzzle over again and solve it (without error) until either (a) I have filled in every square John has, and with the same numbers, in which case John hasn't erred; or (b) I have filled in some square John has, but with a different number, in which case John has erred.

Is there another way to determine whether John has erred? Specifically, is there a way that doesn't involve redoing or continuing the solution, but, rather, involves merely examining what John has done?