The New Sudoku Players' Forum

[tso]

I assume that the rule can't be used in a puzzle where the diagonals count if 1 of the 4 cells is on the diagonal.

Exactly. (I've overlooked this fact while solving a diagonal puzzle more than once.)

[ihope127]

Since at least one of r3c12 must be EITHER 3 or 4 -- it forms a sort of "quantum cell" that contains the possibilities [34].

Ah, the quantum cells. They show just how weird Sudoku can be.

But hey: it is NP-complete. I just got into thinking about a Turing-complete version.