Prelims
a) 10(3) cage at R1C2 = {127/136/145/235}, no 8,9
b) 24(3) cage at R5C3 = {789}
c) 14(4) cage at R7C2 = {1238/1247/1256/1346/2345}, no 9
d) 28(7) cage at R1C1 = {1234567}, no 8,9
1. 45 rule on whole grid 1 innie R7C7 = 7, placed for D\
1a. 28(7) cage at R1C1 = {1234567}, 7 locked for N2
2. Naked triple {789} in 24(3) cage at R5C3, CPE no 7,8,9 in R5C56
2a. 8,9 in N1 only in R2C1 + R3C12, CPE no 8,9 in R6C2
3. 45 rule on N7 2 outies R78C4 = 5 = {14/23}
4. 45 rule on R789 3 outies R6C78 = 12 = {39/48}/[57], no 1,2,6, no 5 in R6C8
5. 45 rule on N12 4(3+1) innies R2C16 + R3C12 = 29
5a. Max R2C1 + R3C12 = 24 -> min R2C6 = 5
5b. 8,9 in N1 only in R2C1 + R3C12
5c. R2C6 = {5689} -> R2C1 + R3C12 = 20,21,23,24 = {389/489/689/789}, no 1,2,5
6. Hidden killer quad 1,2,3,4 in R78C4, 17(3) cage at R7C5 and 37(7) cage at R8C5 for N8, R78C4 contain two of 1,2,3,4, 17(3) cage contains one of 1,2,3,4 -> 37(7) cage must contain one of 1,2,3,4 in N8
6a. 37(7) cage contains three of 1,2,3,4, one is in N8 -> two of 1,2,3,4 must be in 37(7) in N9
6b. Hidden killer quad 1,2,3,4 in 36(7) cage at R6C7 and 37(7) cage for N9, 37(7) cage contains two of 1,2,3,4 in N9 -> 36(7) cage must contain two of 1,2,3,4 in N9
6c. 36(7) cage must contain three of 1,2,3,4, two are in N9 -> one must be in R6C78 -> R6C78 (step 4) = {39/48}, no 5,7
6d. 36(7) = {1345689} (only remaining combination), no 2, 1,5,6 locked for N9
7. 2 in N9 only in R8C78 + R9C7, locked for 37(7) cage at R8C5, no 2 in R8C5 + R9C456
7a. 37(7) cage contains 2 = {1246789/2345689}, 6 locked for N8
8. 45 rule on N6789 3 innies R45C7 + R4C9 = 18 = {279/369/468/567} (cannot be {189/378/459} which clash with R6C78, no 1 in R45C7 + R4C9
8a. 30(7) cage at R1C7 must contain 1, locked for N3
9. 45 rule on N3 2 outies R2C6 + R4C9 = 1 innie R3C7 + 3
9a. Min R2C6 + R4C9 = 7 -> min R3C7 = 4
9b. Max R2C6 + R4C9 = 12, min R2C6 = 5 -> max R4C9 = 7
10. 45 rule on N78 3 outies R8C78 + R9C7 = 14 = {239/248}
10a. Whichever of 3,9 or 4,8 is in R8C78 + R9C7 must also be in R6C78 -> grouped X-Wing for 3,9 or 4,8 in R6C78 and R8C78 + R9C7 -> either 3,9 or 4,8 must be in R123C9
10b. 30(7) cage at R1C7 must contain one of 8,9, which must be in R123C9 -> no 8,9 in R1C78 + R2C6
10c. R2C6 = {56} -> R2C1 + R3C12 (step 5c) = 23,24 = {689/789}, no 3,4 in R2C1 + R3C12
11. 28(7) cage at R1C1 = {1234567} -> whichever of 5,6 is in R2C6 must also be in R1C1 + R3C3
11a. R2C1 + R3C12 + R2C6 = {789} + 5 (cannot be {689} + 6 which clashes with R1C1 + R3C3 containing 6, maybe this can be called an unusual killer combo clash) -> R2C6 = 5, R2C1 + R3C12 = {789}, locked for N1
11b. Naked triple {789} in R2C1 + R3C12, CPE no 7 in R6C2
11c. 45 rule on N2 3 outies R1C1 + R2C2 + R3C3 = 11 containing 5 = {245}, locked for N1, 28(7) cage and D\
11d. R2C5 + R3C456 = {1367}, locked for N2
11e. 2 in N9 only in R89C7, locked for C7
11f. R9C4 = 5 (hidden single in C4)
12. 37(7) cage at R3C7 = {1246789/1345789/2345689}
12a. R345C7 cannot contain more than one of 8,9 (step 10a) -> must contain at least one of 8,9 in N5
12b. Killer triple 7,8,9 in 37(7) and R56C4, locked for N5, 7 also locked for C4, N5 and 24(3) cage at R5C3, no 7 in R5C3
12c. 37(7) cage = {2345689} (only remaining combination), no 1
12d. 1 in N5 only in R4C45 + R5C5, locked for 34(7) cage at R3C2, no 1 in R4C23 + R5C2
13. 1 in N4 only in 39(7) cage at R2C1 = {1356789}, no 2,4
13a. 2,4 in N4 only in R4C23 + R5C2, locked for 34(7) cage at R3C2, no 2,4 in R4C5
13b. 4 in N5 only in R456C6 + R6C5, locked for 37(7) cage at R3C7, no 4 in R345C7
14. R2C6 + R4C9 = R3C7 + 3 (step 9), R2C6 = 5 -> R3C7 = R4C9 + 2
14a. R3C7 = {5689} -> R4C9 = {3467}
14b. 30(7) cage at R1C7 must contain 2, locked for N3
14c. R123C9 must contain either 3,9 or 4,8 (step 10a)
14d. 18(3) cage at R2C7 = {369/468/567} (cannot be {378/459} which clash with R123C9), 6 locked for N3, clean-up: no 4 in R4C9
14e. 30(7) cage contains 4, locked for N3
14f. 18(3) cage = {369/567}, no 8
[Only just spotted …]
15. 7 in R1 only in R1C89, locked for N3 and 30(7) cage at R1C7, no 7 in R4C9
15a. 18(3) cage at R2C7 (step 14f) = {369} (only remaining combination), locked for N3
15b. 30(7) cage contains 3 -> R4C9 = 3, R3C7 = 5 (hidden single in N3), placed for D/, clean-up: no 9 in R6C78 (step 4)
15c. R1C1 = 5 (hidden single in N1)
16. Naked pair {48} in R6C78, locked for R6, N6 and 36(7) cage at R6C7
16a. Naked pair {69} in R45C7, locked for C7, N6 and 37(7) cage at R3C7 -> R2C7 = 3, R6C6 = 3, placed for D\
16b. Naked pair {69} in R23C8, locked for C8 -> R8C8 = 8
16c. Naked pair {16} in R4C4 + R5C5, locked for N5, D\ and 34(7) cage at R3C2 -> R4C5 = 5, R6C5 = 2, R45C6 = [84], 8 placed for D/, R9C9 = 9
17. 39(7) cage at R2C1 (step 13) = {1356789} -> R5C1 = 3
18. R9C4 = 5 -> 37(7) cage at R8C5 = {2345689}, no 1,7 -> R9C6 = 6, R89C5 = [93], clean-up: no 2 in R78C4 (step 3)
18a. Naked pair {14} in R78C4, locked for C4 and N8 -> R4C4 = 6, R5C5 = 1, placed for D/, R3C4 = 3
19. 34(7) cage at R3C2 contains 1,5,6 = {1245679} (only remaining combination), no 8
20. 2,4 in C1 only in R789C1, locked for N7 and 36(7) cage at R7C1 -> R8C4 = 1
20a. R7C2 = 1 (hidden single in N7), R7C4 = 4 -> R7C3 + R8C2 = 9 = {36}, locked for N7 and D/
20b. Naked pair {78} in R9C23, locked for N7 -> R8C3 = 5
20c. Naked pair {24} in R89C1, locked for C1 -> R7C1 = 9
21. R2C8 = 9, R6C4 = 7, both placed for D/, R7C5 = 8, R1C5 = 4, R1C9 = 2, placed for D/
and the rest is naked singles, without using the diagonals.