Can be solved using only Subsets:
***********************************************************************************************
*** SudoRules 20.1.s based on CSP-Rules 2.1.s, config = SFin
*** Using CLIPS 6.32-r779
*** Download from:
https://github.com/denis-berthier/CSP-Rules-V2.1***********************************************************************************************
singles ==> r1c6 = 7, r2c6 = 1
237 candidates, 1680 csp-links and 1680 links. Density = 6.01%
whip[1]: c6n8{r9 .} ==> r9c5 ≠ 8, r7c5 ≠ 8
hidden-triplets-in-a-block: b9{n3 n6 n7}{r9c8 r9c7 r7c8} ==> r9c8 ≠ 9, r9c8 ≠ 8, r9c8 ≠ 5, r9c8 ≠ 4, r9c7 ≠ 9, r9c7 ≠ 5, r7c8 ≠ 9, r7c8 ≠ 8, r7c8 ≠ 5, r7c8 ≠ 4
swordfish-in-columns: n8{c1 c6 c9}{r4 r9 r7} ==> r9c3 ≠ 8, r9c2 ≠ 8, r7c3 ≠ 8, r7c2 ≠ 8, r4c8 ≠ 8, r4c3 ≠ 8
swordfish-in-columns: n2{c1 c6 c9}{r1 r9 r6} ==> r9c5 ≠ 2, r9c4 ≠ 2, r9c3 ≠ 2, r6c8 ≠ 2, r6c5 ≠ 2, r1c8 ≠ 2, r1c5 ≠ 2, r1c3 ≠ 2
whip[1]: c5n2{r3 .} ==> r2c4 ≠ 2, r3c4 ≠ 2
naked-single ==> r3c4 = 6
naked-pairs-in-a-block: b2{r1c5 r2c4}{n4 n5} ==> r2c5 ≠ 5, r2c5 ≠ 4
swordfish-in-rows: n4{r2 r5 r8}{c8 c4 c3} ==> r9c4 ≠ 4, r9c3 ≠ 4, r7c3 ≠ 4, r4c4 ≠ 4, r4c3 ≠ 4, r1c8 ≠ 4
jellyfish-in-columns: n9{c1 c9 c5 c6}{r9 r7 r6 r4} ==> r9c3 ≠ 9, r9c2 ≠ 9, r7c3 ≠ 9, r7c2 ≠ 9, r6c8 ≠ 9, r6c3 ≠ 9, r6c2 ≠ 9, r4c8 ≠ 9, r4c7 ≠ 9, r4c3 ≠ 9
naked-quads-in-a-block: b7{r7c2 r7c3 r9c2 r9c3}{n5 n6 n7 n1} ==> r9c1 ≠ 1, r8c3 ≠ 5, r8c2 ≠ 5
hidden-quads-in-a-column: c3{n2 n8 n4 n9}{r2 r3 r5 r8} ==> r5c3 ≠ 7, r3c3 ≠ 1, r2c3 ≠ 3
whip[1]: r3n1{c8 .} ==> r1c8 ≠ 1
hidden-quads-in-a-column: c8{n2 n4 n8 n9}{r3 r2 r8 r5} ==> r8c8 ≠ 5, r5c8 ≠ 7, r3c8 ≠ 1, r2c8 ≠ 5, r2c8 ≠ 3
hidden-single-in-a-block ==> r3c7 = 1
x-wing-in-rows: n5{r2 r8}{c4 c7} ==> r9c4 ≠ 5, r4c7 ≠ 5
stte