1 20 17 12  3
 16 11  2  7 18
 21 24 19  4 13
 10 15  6 23  8
 25 22  9 14  5
41 invocations of solve

  1 28 33 20  3 26
 34 19  2 27  8 13
 29 32 21 12 25  4
 18 35 30  7 14  9
 31 22 11 16  5 24
 36 17  6 23 10 15
5422 invocations of solve

  1 28 37 24  3 26 17
 36 39  2 27 18 11  4
 29 42 23 38 25 16  9
 40 35 30 19 10  5 12
 43 22 41 32 15  8 47
 34 31 20 45 48 13  6
 21 44 33 14  7 46 49
11272348 invocations of solve

  1 38 55 34  3 36 19 22
 54 47  2 37 20 23  4 17
 39 56 33 46 35 18 21 10
 48 53 40 57 24 11 16  5
 59 32 45 52 41 26  9 12
 44 49 58 25 62 15  6 27
 31 60 51 42 29  8 13 64
 50 43 30 61 14 63 28  7
3242065 invocations of solve