In case you did not see, Santa Klaus posted one additional yesterday.
Here is the link again:

is.gd/648_one_more_str8t

didnt like this one, too many candidates. additional D3=6 makes it quite normal.

#649 Weekly Extreme: very difficult.

Spoiler (!; if you don’t want to use the Klaus’ hint):
After (V)CE, Settis on 5, 2, 4 and 2 URs (G7; HJ4), I checked 5 in B8 (→ B9 empty); then another UR (C6 solved); EF4 hi/lo (→ via a chain with using Settis, no 8 in row B); finally third chainy issue with a beautiful triple hidden triple [789] in cols 2, 6 and 9 colliding with B4 when F2 or F6 ≠ 7; the rest is about possible URs and/or Y-wings and so…

Sorry, I was blind. No chain needed! Settis 5, 4, 2 (this especially), 8, but then works 7+9 Setti consideration. URs, small fishes and one Y-wing inbetween…

@kozibrada: Great work! Thanks for sharing your ideas.
For all who struggle to follow kozibradas solution path: Here with a bit more details (and an open issue that kozibrada might comment on :) )

1) UR HJ4
2) CE: remove 7 from E8
3) setti 2: if 2 would be missing in any col then in col 8 which would automatically lead to row D missing 2.
4) Hence, all other rows than D must definitely hold a 2 -> A12 = (23) and we can remove 2 from BCE8
5) setti 5 -> G9=5
6) setti 8
7) naked quadruple in col 4
8) setti 4 in row A
9) HC: if D8=4 -> E8=3 and setti 9 (combined with setti 7): all rows hold a 9 except rows A and G -> D2=9 -> D1=7 -> D3=8 -> E1345 all hold numbers (456) contradiction; hence D8=2
10) setti 4 in row D
11) UR G7=2
@kozibrada: I don't see how to apply a combined setti 7/9 argument here already (?)
12) chain to checked if row D misses a 6 and hence holds a 9 -> D1-3 = (678)
13) combined setti 7/9: rows E and F must both hold 7 and 9
14) setti 7: A4=9
15) 2 wing on 2 (HJ45)
16) 2 wing on 4 (AB89)

In the above solution path the cahin can be avoided, if we replace steps 12-16 by the following steps 12' - 15':

12') HC: F=9 (again using the setti 7/9 arguement leads to the same contradiction as above)
13') Consideration: D9=9 would lead to the fact that row E misses 9. If D9!=9 row D misses 9. With the combined setti 7/9 argument we know that row F holds 7 and hence misses 1.
14') 3 fish on 7 in rows DEF
15') 2 fish on 4 in AB89

This also answers my own question to kozibrada.

Slightly different in the middle:
D2=9->B6=9->B4=7
D2!=9->x-wing 9 col 2+6->B4=7

@Toni: Thamk you for your solution.

A little bit another interpration of the 7+9 Setti consideration.
As shown by you: If there is a 9 in row D (D2=9), there can’t be a 9 in row E.
This means, there are 2 cols (5 and 7) and 2 rows (G and (D or E)) for sure without 9.
A4=7 => no 9 in row A and col 8, H8=7
A4=9 => no 7 in row A and col 8 => J8=9

This leads to the following consequences:
7 and 9 are a must in row F
No 7 in AE8 (=>D8=2)

In the end, you only need your 2 fish on 4 in AB89.

Hello, could anyone explain what a CE is?
I tried googling the strategy but can't find anything about it.
Thanks in advance!

(V)CE stands for "(Very) Careful Elimination". It is not a special technique. It is just checking combinations of candiadates mostly in small local area/compartment e.g. within a row or col. E.g. if you have a compartment of 3 cells and one of them has the only remaining candidates (47), then you can remove 4 and 7 from the other cells in that compartment by "CE". If you refer to my step 2 above, this means the interaction between cells E89 and J89.

Unfortunately there are no simple examples for CE. The one above is a big gap

Actually, Tony's step 2 is a good example for CE: E8 could be 7 at face value. However, at more careful inspection, it leads to conflict quickly: E9=8, J9=9, J8=8, H8=7. E8!=7 through CE.

Oh I understand now, thank you for the detailed explanation!

i would say typically you get CE in rows/columns with three compartments, where the interaction of all three compartments sometimes gives elimination you would not get if you just look at one or two of the compartments.

is.gd/kst_649_BX

very nice one - not difficult - no chain - thnx

Easy compared to other puzzles, but still nice. Only x-power, b-power an settis needed.

Agree with witch and Martinus, enjoyable solve.

agree with the above, nice, fun, thanks

Agree with all others. Nice, lots of fun.

is.gd/kst_649_B

Very easy.

great fun

Settis are the key. Thanks!

Nice, straightforward, not difficult. Thanks, kst.

is.gd/kst_649_X

Definitely the hardest one of the additional kst puzzles this week. Probably there are more elegant ways to solve it than with my path (maybe it still inspires others to find their way...):

1) standard x eliminations
2) Setti 4
3) Setti 8
4) combined setti 7/8 in row H --> removes 5 from H12
5) CE: remove 5 from D4, F5, F6
6) standard x eliminations (especially diag)
7) easy HC: remove 8 from A1
8) easy HC: remove 8 from J3
9) easy HC: remove 4 from A1
10) CE in A-C6
11) more complicated HC: remove 8 from J9
12) CE: remove 4 from E5
13) chain on the remaining 2 alternatives 3 and 8 in A6 --> A6=3
14) gap in D1-5
15) G7=5 single in diag
16) HC:remove 8 from F6

Thank you, kst!

Instead of the step 13), you can look at 1 in G6 (a straight conflict with J9) → Setti on 1 = no 1 in rows D, E, H + sure 1 in col. 1; also we know all Settis → CE in row H. Skewed 2-fish on 4 (FJ4AJ9) → C7, G3 ≠ 4, J2567 ≠ 4. Very skewed 2-fish on 4 (CF3AJ9) → F4 ≠ 4. Step 16): you surely meant ‘7 from F6’; it can be solved by an X-cycle on 7 if H2 = 7: this cycle contains seven members (D34F6G3H2J26; an odd number), which is wrong → H2 ≠ 7.

Challenging puzzle, indeed. Thanks for your ideas, Tony, I got stuck between steps 8 and 9. With fresh inspiration from your notes, I found
11') J9!=4 (otherwise F4=4 --> C7=1 but also D3=4, D7=1)
speeding things up before kozibrada's Setti 1.

kozibrada's beautiful sequence of fish-on-4 does the same thing, I guess, expect it's prettier. Thumbs up! There's just a typo in the second fish, if I'm not mistaken: it should be CF3J49, shouldn't it?

As last step, instead of H2!=7, I had J2!=78 as inspired by the cycle of bi-value cells 78. Comes down to the same idea, though.

Thanks to the "usual suspects" :) to improve my path. Great ideas! With all that head chaining I think that it is still among the most complciated puzzles of the last weeks/months

@Rübennase: Yep, first I found J9 ≠ 4 via mid-chain, where ABC6 = hi → F6G7 = [56] → E5 = 89 → F4 = 5 → J4 = 4 – conflict with J9. Your path is quicker, very well. But then I noticed the fishes on 4.
No typo, the CF3AJ9 fish works as well (F3 or A9 = 4), but I admit the fish involving the row J is clearer. There is actually another X-cycle (on 4, with five members – C3F34J49) → F4 ≠ 4.

Thanks, kozibrada! You're right, of course. I wish I could see skewed fish like you can. Need to bend my eyes a bit, I guess ;)

is.gd/kst_649

Very nice one. After some standard CE, an easy to spot SI in the upper right corner opens the door. Then a (hidden) UR lets the puzzle collapse. [There are even more hidden URs that were not needed]

Actually no UR is needed at all after the SI door opener mentioned above. But there are many UR options to find...

Very nice. Thanks, kst!