19
29
39
49
56
56
56
56
56
Hidden Quad
The four-cell version of a hidden subset: four digits restricted to the same four cells reserve them, so all other candidates leave those cells.
There's a move in row 1.
The four-cell version of a hidden subset: four digits restricted to the same four cells reserve them, so all other candidates leave those cells.
There's a move in row 1.