Continued research into the issue of Anti-Quadruples has exposed why we had not seen them before. They are best detected not by observing them directly, but by observing what isn’t there.
Rules:
Normal Sudoku rules apply.
Digits within colored circles do not appear in cells touching the circle.
The marked diagonals each contain the digits 1 to 9 exactly once.
Estimated difficulty: 2 out of 5
Solve on SudokuPad