Nonet Layout:
122222334
112223344
511233444
551133446
555113466
557788666
577798866
777999886
799999888
Prelims
a) 11(2) at R2C2 = {29/38/47/56} (no 1)
b) 7(2) at R2C3 = {16/25/34} (no 7..9)
c) 15(2) at R3C3 = {69/78} (no 1..5)
d) 9(3) at R3C4 = {126/135/234} (no 7..9)
e) 19(3) at R3C5 and R8C5 = {289/379/469/478/568} (no 1)
f) 14(2) at R3C9 = {59/68} (no 1..4,7)
g) 5(2) at R4C1 = {14/23} (no 5..9)
h) 26(4) at R4C2 = {2789/3689/4589/4679/5678} (no 1)
i) 9(2) at R4C7 = {18/27/36/45} (no 9)
j) 12(2) at R5C8 = {39/48/57} (no 1,2,6)
k) 8(2) at R6C6 = {17/26/35} (no 4,8,9)
l) 11(3) at R8C9 = {128/137/146/236/245} (no 9)
1. 1 in N5 locked in R34567 for C1
2. 1 in N1 locked in R45C4 for C4
2a. 9(3) at R3C4 (prelim d) = {1..} = {126/135} (no 4)
2b. cleanup: no 6 in R2C3
3. Innies N2: R1C2+R3C4 = 7(2) = [16/25/43/52] (no 3,6..9 in R1C2)
4. Outies N1234: R3C1+R4C9 = 9(2) = [18/36/45] (no 2,7,9; no 5,6,8 in R3C1)
4a. cleanup: no 5 in R3C9
5. Innie/outie difference (IOD) N123: R3C1 = R1C8
5a. -> R1C8 = {134}
6. IOD N3: R5C5 = R1C8 + 1
6a. -> R5C5 = {245}
6b. 19(3) at R3C5 (prelim e) = {(2/4/5)..} = {289/469/478/568} (no 3)
6c. can only contain 1 of {245}, which must go in R5C5
6d. -> no 2,4,5 in R34C5
7. Innies N6: R4C9+R6C7+R8C9 = 11(3) = {128/146/236/245} (no 7)
(note: {137} blocked because none of these digits in R4C9)
7a. only 1 of 5,6,8, which must go in R4C9
7b. -> no 5,6,8 in R6C7+R8C9
7c. cleanup: no 1,2,3 in R6C6
Now for what's probably the most complicated move I've ever made in a walkthrough...
8a. Law of Leftovers (LoL) C89: R3456C7 (outies) = R189C8+R9C9 (innies)
8b. R3456C7 (being all peers of each other) cannot contain any repeats
8c. -> R189C8+R9C9 cannot contain any repeats either
8d. IOD C89: R3C7 = R8C8
8e. -> R3C7 maps to R8C8
8f. The remaining three outies R456C7 must map to R19C8+R9C9
8g. if R6C7 = R1C8, the same digit would be forced into R9C9**, which is impossible (see step 8c)
(**Reason: R12C9 could not contain the digit because they are peers of R1C8 (same cage),
R45678C9 could not contain the digit because they are peers of R6C7, and
R3C9 could not contain the digit because it has no candidates in common with R6C7)
8h. -> R6C7 must map to one of R9C89
8i. -> 11(3) at R8C9 and 11(3) at R4C9+R6C7+R8C9 (step 7) (which both share the cell R8C9)
must contain the same combination!
8j. -> R4C9 = R9C8 (the only non-peer of R4C9 in 11(3) at R8C9)
8k. -> R9C8 = {568}
8l. -> R6C7 = R9C9
8m. -> R9C9 = {123}
8n. -> the remaining two outies from step 8a, R45C7 (= 9(2)), must map to R19C8
8o. -> R19C8 must also sum to 9
8p. Outies C9: R179C8 = 15(3)
8q. -> R7C8 = 6
(from outie cage split using R19C8 = 9(2) from step 8o)
8r. -> R45C7 must contain 1 of {58} (step 8n)
8s. -> 9(2) at R45C7 = {18/45} (no 2,3,6,7)
8t. cleanup: no 8 in R3C9, no 6 in R3C7 (step 8e), no 3 in R3C1 (step 4), no 3 in R1C8 (step 5), no 4 in R5C5 (step 6)
Grid state after step 8:
Code:
.-----------------------.-----------------------------------------------.-----------.-----------------------.
| 23456789 1245 | 123456789 23456789 123456789 123456789 | 123456789 | 14 123456789 |
| .-----------+-----------------------. .-----------' :-----------. |
| 23456789 | 23456789 | 12345 23456 | 123456789 | 123456789 123456789 | 12345789 | 123456789 |
| | :-----------.-----------+-----------+-----------.-----------' :-----------:
| 14 | 23456789 | 6789 | 2356 | 6789 | 123456789 | 12345789 12345789 | 69 |
:-----------+-----------: | | | :-----------. | |
| 1234 | 23456789 | 6789 | 12356 | 6789 | 123456789 | 1458 | 12345789 | 58 |
| | '-----------: | | | :-----------+-----------:
| 1234 | 23456789 23456789 | 12356 | 25 | 123456789 | 1458 | 345789 | 12345789 |
:-----------: .-----------'-----------+-----------+-----------'-----------: | |
| 123456789 | 23456789 | 123456789 23456789 | 123456789 | 567 123 | 345789 | 12345789 |
| '-----------: .-----------' '-----------------------+-----------' |
| 12345789 12345789 | 12345789 | 2345789 12345789 12345789 12345789 | 6 12345789 |
:-----------. | :-----------.-----------------------.-----------'-----------.-----------:
| 23456789 | 123456789 | 123456789 | 23456789 | 23456789 23456789 | 123456789 12345789 | 1234 |
| '-----------'-----------' | .-----------' .-----------' |
| 23456789 123456789 123456789 23456789 | 23456789 | 123456789 123456789 | 58 123 |
'-----------------------------------------------'-----------'-----------------------'-----------------------'
9. R45C1 (prelim g) = {23}, locked for C1 and N5
(Note: {14} blocked by R3C1)
10. R4C9+R6C7+R8C9 (step 7) and 11(3) at R8C9 (step 8i) = {128/245} (no 3)
10a. 2 locked in R89C9 for C9
10b. cleanup: no 5 in R6C6
11. 19(3) at R3C5 (step 6b) = {289/568} (no 7)
11a. 8 locked in R34C5 for C5 and N3
12. 8 in N2 locked in R1C346 for R1
13. Innies R89: R8C23 = 7(2) = {16/25/34} (no 7..9)
13a. -> R78C2 cannot sum to 7 (combo crossover clash (CCC))
13b. -> R67C1 (remaining two cells of 17(4) at R6C1) cannot sum to 10
13c. -> R3C1 (remaining innie cell in N5) cannot sum to 4 (= 45 - 10 - 5 - 26)
13d. -> no 4 in R3C1
14. Naked single (NS) at R3C1 = 1
14a. -> R4C9 = 8 (step 4), R1C8 = 1 (step 5), R5C5 = 2 (step 6)
14b. -> R3C9 = 6, R45C1 = [23], R9C8 = 8 (step 8j), R34C5 = [89] (step 11)
14c. -> R34C3 = [96] (prelim c, last permutation), R45C7 = [18] (step 8n)
14d. -> R6C67 = [62]
14e. -> R9C9 = 2 (step 8l)
14f. -> R8C9 = 1 (cage sum)
14g. cleanup: no 5 in R23C2, no 3 in R2C2, no 4 in R56C8, no 9 in R6C8, no 9 in R8C8 (step 8e)
15. Hidden single (HS) in C4 at R5C4 = 1
15a. -> split 8(2) at R34C4 = {35} (last combo), locked for C4
15b. cleanup: no 2,4 in R2C3, no 5 in R1C2 (step 3)
16. 4 in N1 locked in R12C1+R23C2
16a. -> no 4 in R1C2 (CPE)
17. NS at R1C2 = 2
17a. -> R3C4 = 5 (step 3)
17b. -> R4C4 = 3
17c. cleanup: no 8 in R2C2, no 5 in R8C8 (step 8e)
18. 15(3) at R3C6 = {357} (last combo)
18a. -> R3C6 = 3, R45C6 = {57}, locked for C6 and N3
18b. cleanup: no 3 in R8C8 (step 8e)
19. Naked pair (NP) at R2C47 = {46}, locked for R2
All singles and cage sums to end.