% obviously unsolvable
..78.64..
.465...8.
2...1..37
4.4.9..56
..92.73..
51..4...9
69..8...3
.2...597.
..54.92..
% really hard
.5.....6.
..4.2.3..
9...4...5
.96...5..
...4.8...
..5...13.
7...6.2.3
..2.1.9..
.3.....7.
% unsolvable
.5.....6.
..4.2.3..
9...4...5
.96..15..
...4.8...
..5...13.
7...6.2.3
..2.1.9..
.3.....7.
% empty: surely solvable, but surprisingly hard (large search space)
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% symmetry breaking helps a little: morally, this is still empty
123......
456......
789......
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