Walls, extensions, and frameworks

Bill Spight · #1

Yeah, I know it's a nebulous topic. It arises from some recent discussions here, that I want to sort of tie together.

There is a well known proverb about extending (on the third line) from a single stone or a simple wall, in order to make a base. But what about extensions from walls in general?

If a wall is strong enough, it does not need an extension. But if you do not extend, how do you make use of it?

Often pros make extensions that are longer than the proverb says. What is going on there?

What about extending from weak walls, or walls with defects? Or do you not extend, but repair the defect?

I cannot claim any expertise about these questions, but I think that they leave most go players puzzled. So I thought that this would make a good study topic, even though we are unlikely to come up with anything definitive.

Bill Spight · #2

Since building frameworks is an important aspect of long extensions, I thought I would use some pro games from the New Fuseki era. But I found that many of the ideas that I had thought came from the New Fuseki arose earlier.

Here is an awesome game from 1930 between Honinbo Shusai Meijin and Go Seigen.

I found the Meijin's center oriented play quite impressive.

Chew Terr · #3

Thanks, that's a really interesting game! I've been thinking a lot about your comments regarding walls that don't need extensions and uses of thickness, and intend to try to use them more, myself. Between that, and a few other ideas I have taken from your comments (for example, you seem to recommend long knight approaches to corners in proper situations more than most people I talk to), I intend to experiment and learn from some of your ideas. However, things like not extending from walls is complicated enough that I figure I'm likely to lose two stones until I master it, which may take a long time. If I come up with any good questions or results from my experimentation, I'll try to post them here.

Bill Spight · #4

Chew Terr wrote:

You seem to recommend long knight approaches to corners in proper situations more than most people I talk to,

The idea of using that approach to make for an easy opening (for yourself) is one that I picked up from Go Seigen's recent writings. My first reaction was that maybe he is an old man still thinking like he did in pre-komi days. But now I think that when a one space approach makes a pincer a good move, and also gives the opponent a choice of good moves, the long knight approach makes a lot of sense.

Quote:

However, things like not extending from walls is complicated enough that I figure I'm likely to lose two stones until I master it, which may take a long time.

You may be in for a pleasant surprise.

When you have a strong wall that does not require an extension, often the way to make use of it is to play all out in other regions of the board.

Chew Terr · #5

Bill Spight wrote:

You may be in for a pleasant surprise.

When you have a strong wall that does not require an extension, often the way to make use of it is to play all out in other regions of the board.

As I tend to play for thickness often, and tend to play too aggressively a lot, perhaps it will work out for me, after all. =D

Numsgil · #6

So I spent the weekend re-reading Wilcox's sector fights, where he spends a lot of effort talking about walls. I think I can make this a bit more productive of a conversation by helping to define some terms a little better. I'm going to present this information as authoritative, but it really only represents my novice understanding

Anyway, there are three kinds of plays that might be thought of as extensions:
1. Extension to form eye space (AKA safety extension)

If your wall is relatively flat, and you made it in sente, and it faces a neutral region of the board, a move at 1 here is urgent. No other move is proper. Likewise a white move at 1 will severely attack the black wall (white might be able to inch one move closer, too). So you never ever ever want to build a flat wall in gote facing a neutral region like this.

Click Here To Show Diagram Code: [go]$$B $$ +---------------------------------------+ $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . O X . . . . . . . . . . . . . . . | $$ | . . O X . . . . 1 . . . . . . . . . . | $$ | . . O X . . . . . , . . . . . , . . . | $$ | . . O X . . . . . . . . . . . . . . . | $$ | . . O . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . |[/go]

Black builds a flat wall in gote, but it faces an existing stone. Black builds his wall up only as high as necessary to make the extension the proper distance. Black takes sente after 9 and can play elsewhere (yes, the contact exchange is artificial, but the point is walls, not contact fighting). Black should not keep responding to white, as he'll probably end up over concentrated. If black builds a larger wall in sente, he can probably attack the corner in the top right (covered next), since his wall becomes thick, but it becomes a game of chicken to see who takes sente first. Whoever gets sente first will probably want to play at 'a'. And the existing extension stone probably becomes a bit badly placed.

Click Here To Show Diagram Code: [go]$$B $$ +---------------------------------------+ $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . 6 7 . . . . . . . . . . . . . . . | $$ | . . O 1 . . . . X . . . . a . . . . . | $$ | . . 2 3 . . . . . , . . . . . O . . . | $$ | . . 4 5 . . . . . . . . . . . . . . . | $$ | . . 8 . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . |[/go]

2. Extensions to form a moyo

A 3-3 invasion joseki took place on the left side. Black has built his wall in gote, and it faces an empty side and white symmetric corner. Black's wall is not flat. It doesn't need an extension for eyes, as it should be able to form 2 eyes with out much trouble if the need arose.

It's a wall that doesn't need an extension. At least not for eyes. But a white move at 'a' would still be great. Urgent even, as it would make black's wall overconcentrated. Black can't effectively pincer the stone, as his thickness is a bit too far away to really come to bear with its full strength.

Suppose white then decided to play a point on the lower side for whatever reason. It's black's sente. He could play 'a', and it wouldn't even really be a bad play. But he doesn't have to play a. He's stong on the left, so he can nestle right up to white's weak corner and form a moyo. Black welcomes a pincer, as it forms a big strong-weak-weak-weak sandwich, with black in the middle. He should be able to attack and profit. If white plays a more standard retreat joseki, black has built up a moyo on the top. If white tries to invade at 'a' now, he has a weak group sandwiched between two strong groups. White is not going to like that fight. Again, black should be able to attack and profit. If the corner to the right was strong, black should probably hold back to 'a'.

Click Here To Show Diagram Code: [go]$$B $$ +---------------------------------------+ $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . O O X . . . . . . . . . . 3 . . . | $$ | . . O X . X . . . a . . . 1 . . 4 . . | $$ | . . O X . . . . . , . 5 . . . O . . . | $$ | . O X . . . . . . . . . . . . . . . . | $$ | . O X . . . . . . . . . . . . . 2 . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . |[/go]

For comparison, in this position, '1' is an overplay, because black's wall has no eye space. It's a wall, yes, but it's also a weak group. An invasion at 2 acts as an attack on the wall and a pincer against 1. It's a move with double purpose. Black will have a hard time in this fight, as it's a strong-weak-weak-weak-weak group sandwich, but white is in a clear position to profit on the right and left sides, and black can only really profit if he can find an effective way to attack the middle white group, and that's unlikely, since any move black makes against it just induces white to escape, and thereby attack the other side.

In fact, white might even use 2 as a sacrifice. He can force black to gobble it up (maybe white can make life locally, maybe he can't), but white builds massive influence that will probably coordinate well with the bottom. Normally black should be the one to transfer his wall, but here white can turn black influence into a white wall.

Click Here To Show Diagram Code: [go]$$B $$ +---------------------------------------+ $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . O X . . . . . . . . . . . . . . . | $$ | . . O X . . . . . 2 . . . 1 . . . . . | $$ | . . O X . . . . . , . . . . . O . . . | $$ | . . O X . . . . . . . . . . . . . . . | $$ | . . O . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . |[/go]

3. Extensions which attack

If your wall is thick, you can also use it to invade a thin enemy position.

In this position, white had an approach-ish move against the corner, but then decided to invade, and they picked this joseki (not sure if it's a good idea, but let's say we're here now and the board is what it is). White played elsewhere. Black's sente now. White had an unusual moyo framework along the top, and it now comes under attack by black's wall. Either black 1 is too far to be a safety extension. But it is a good invasion point once white's moyo can't grow along the side anymore. So black plays 1 to invade. Probably white will escape and live. That's fine. Black's wall has prevented white from being too aggressive in attacking the invasion. And black has destroyed much of white's moyo, and has a new wall that threatens a bit of the white corner even. Black has used his wall for attack and is happy enough with the result (although he better hope he has some territory on the bottom, as he didn't make any on the top!)

Click Here To Show Diagram Code: [go]$$B $$ +---------------------------------------+ $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . O O X . . . . . . . . . . . . . . | $$ | . . O X . X . . O . 1 1 . . . . . . . | $$ | . . O X . . . . . , . . . O . O . . . | $$ | . O X . . . . . . . . . . . . . . . . | $$ | . O X . . . . . . . . . . . . . . . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . |[/go]

For comparison, black's wall here can't really be used to attack very effectively. Black and white both run out, but it doesn't take long before black's wall becomes just another weak group, and since white is the center weak group between two black weak groups, the fighting will probably favor white.

Click Here To Show Diagram Code: [go]$$B $$ +---------------------------------------+ $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . O X . . . . . . . . . . . . . . . | $$ | . . O X . . . O . 1 . . . . . . . . . | $$ | . . O X . . . . . , . . . O . O . . . | $$ | . . O X . . . . . . . . . . . . . . . | $$ | . . O . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . |[/go]

4. Useless wall

Chew Terr · #7

Numsgil: For your "useless wall", while it is probably the wrong direction and certainly sub-optimal, I figured it might be worth noting that the wall does still provide good potential opportunities around 'a'.

Numsgil · #8

Chew Terr wrote:

Numsgil: For your "useless wall", while it is probably the wrong direction and certainly sub-optimal, I figured it might be worth noting that the wall does still provide good potential opportunities around 'a'.

I dunno...

If that were a white moyo, the wall could be used to back up reductions or invasions, so it would be a bit inefficient, but salvageable in terms of value. But that's white territory. Black caps. So what? White wasn't looking to create a moyo there anyway (that's why he played so low). Plus it doesn't really threaten a follow up (I don't think), so it's gote.

He could try to attach somewhere, but it's going to be a pretty feeble reduction of white territory. White will just connect along the second line if it's at the 'a' you show. If it's further away, white should be able to capture it I think. Or give up a few points of territory for thickness and influence of his own.

Black's cap might aim at a central moyo, but that's going to be too leaky to really become secure territory. Black could cap to form more central thickness with an eye to attacking some groups on the bottom(which is arguably what he should be doing at this point), but it's still gote and slow.

I don't know... you'd have to be pretty clever to make that black wall non-useless. And at best it's too many stones for not enough work. It's profile to the rest of the board (maybe 'shadow' would be a better term) is only like 3-4 stones wide. That's for the right side/bottom right corner. On the left side or bottom left corner it's just 1-2 stones. And on the left side it's undercut, which dramatically reduces any effectiveness.

Of course, if that black wall actually represents an invasion of a weak point in a white moyo on the top left side, then it's probably a good invasion because the invasion was successful (lived) and it escaped into the center. But in terms of being a wall it's still pretty useless.

Chew Terr · #9

Numsgil wrote:

I dunno...

Remember, though, that white has played a LOT more stones up top. Presumably black has had the same opportunity, so while black's strength is certainly partially nullified, black probably does not have to get an equivalent result from his wall in order to be content. 'a1' probably makes framework or territory, depending on the bottom left, and while it's not as much as white has across the top, it goes a fair way depending on what's down there. 'a2' keeps ahead in central influence and makes it harder for white to ever make more than third-line territory up top. 'a3' allows white to solidify somewhat, but takes either a reasonable chunk of white's territory or a tone of thickness.

Again, I may be disagreeing just for the sake of disagreeing, but when I hear absolutes, sometimes I twinge. I understand that your picture was just to demonstrate a general idea, and apologize for making a big deal out of overly specific details of a more general diagram. I'll just cede the point that, at least in a reasonable number of possible bottom-half situations, that wall is mostly neutralized.

emeraldemon · **#10**

Not exactly the same, but I've been thinking about & researching extensions from and against the 3-4 enclosure.

Typically, black wants this formation, +/-1 in any direction:

Click Here To Show Diagram Code: [go]$$Bc 19x19 diagram $$ --------------------------------------- $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . X . . . . . X . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . . . . . . . . . . . . . . . . . . |[/go]

That's a 5 space jump. Using the n+1 rule, it seems to imply the enclosure is worth about the same as a 4-stone wall, which seems a bit of a stretch. It seems like a pretty simple position, but it's amazing all the ways pros intentionally try to get or prevent that extension.

Click Here To Show Diagram Code: [go]$$Wc Note the forced small 6 $$ --------------------------------------- $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . X . 7 . . 1 . 6 . . 2 3 . . . . | $$ | . . . , . . . . . , . . . 4 . O . . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . 5 . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . . . . . . . . . . . . . . . . . . |[/go]

Click Here To Show Diagram Code: [go]$$Wcm1 W stretches to get 3 $$ --------------------------------------- $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . 5 . . . | $$ | . . . X . . . 4 . 3 . . . 1 . . . . . | $$ | . . . , . . . . . , . . . . . X . . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . 2 . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . . . . . . . . . . . . . . . . . . |[/go]

Or sometimes, ignore it completely:

Click Here To Show Diagram Code: [go]$$Bc This one cracks me up $$ --------------------------------------- $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . X . . . . . 1 . . . . . . . . . | $$ | . . . , . . . . . , . . . 3 . X . . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . 7 . . . . . . . . . . 2 . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . 5 . . . . . , . . . . . 4 . . . | $$ | . . . . . . . . . . . . . . . . . . . |[/go]

Full games under the hide, check out the last one especially:

Toge · **#11**

I wanted to ask advice for this basic fuseki pattern, where black seems to get his stones working much more effectively than white. First of all black 3 works really well with black stone on right. If white invades at triangle spot, black can trade it for corner. Approaching at "a" direction seems best for contesting black's influence, but with the black hoshi stone on top right, it's often said to be sub-optimal.

Click Here To Show Diagram Code: [go]$$c $$ --------------------------------------- $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . O . . . . . , . . . . . X . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . 2 . . . . . . . . . . . . . a . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . O . . . . . 3 . . . . . X . . . | $$ | . . . . . 1 . T . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ ---------------------------------------[/go]

hyperpape · **#12**

emeraldemon wrote:

Click Here To Show Diagram Code: [go]$$Bc 19x19 diagram $$ --------------------------------------- $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . X . . . . . X . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . X . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . . . . . . . . . . . . . . . . . | $$ | . . . , . . . . . , . . . . . , . . . | $$ | . . . . . . . . . . . . . . . . . . . |[/go]

That's a 5 space jump. Using the n+1 rule, it seems to imply the enclosure is worth about the same as a 4-stone wall, which seems a bit of a stretch. It seems like a pretty simple position, but it's amazing all the ways pros intentionally try to get or prevent that extension.

I don't think it implies that. A wall is (typically? maybe even always) up against neighboring stones. This enclosure has options both where the extension is and in the corner. In cases where white has supporting stones and can play directly in the corner, black may end up with a more developed wall. If white invades the extension, black may end up with a solid corner and a group to attack.

Drawing conclusions about the enclosure based on the n+1 rule is very hard because the strategic and tactical issues are so different.

Numsgil · **#13**

Chew Terr wrote:

Again, I may be disagreeing just for the sake of disagreeing, but when I hear absolutes, sometimes I twinge. I understand that your picture was just to demonstrate a general idea, and apologize for making a big deal out of overly specific details of a more general diagram. I'll just cede the point that, at least in a reasonable number of possible bottom-half situations, that wall is mostly neutralized.

I don't mind, I like arguing

And I tend to phrase things in terms of absolutes (even if I'm not sure they're absolute) because otherwise it invites wishy-washy language. Present it as absolute and provide some backing evidence and see what the rebuttal looks like. If you make the argument as specific and fragile as possible, it makes the rebuttal easier if it's wrong. If it's right, you have a nice, easy to vocalize rule without fluff. It was ingrained in me when I was in highschool and we were doing AP classes (History especially).

Chew Terr · **#14**

Numsgil wrote:

I don't mind, I like arguing

Great, glad you didn't take it too seriously and whatnot. It's been a long time since high school debate, so I've moved away from using absolutes for the sake of it. (Cue flashbacks to quoting Malthus in Lincoln-Douglas debates to do my best to mess with folks's heads).

Jedo · **#15**

I don't really like :w2:

in that last diagram, because it allows :b3:

If I were white, I'd play some pincer, possibly high so that black can't jump and press down.

Numsgil · **#16**

emeraldemon wrote:

Not exactly the same, but I've been thinking about & researching extensions from and against the 3-4 enclosure.

With the 4-4 stone at least, the extension is where it is because if white invades at the 3-3, the resulting wall for black will be perfectly distanced from the extension. If white invades with an inside approach move, black can force white to overconcentrate (that is, the extension stone acts as a ready-made pincer). Further away and it wouldn't act as a pincer. eg:

Which means that on a huge board, I'll bet when you extend from a 4-4 stone, you don't chose the middle of a side, but chose the 7 point jump from the corner (the middle of a side is more like forming a vast moyo ie: something like niren-sei).

Not sure about the 3-4, but I bet it follows similar logic. It's not about forming the largest moyo possible (if it was, you'd approach the far corner it's facing). It's not about securing territory (if it was you'd make a tighter extension). It's about forming the largest area with a pre-existing pincer stone that will make white reluctant to invade, because he'll be concerned about becoming overconcentrated so early in the fuseki, or just helping black form secure territory. And if you expand to the other side and reinforce both wings, you've just about got a huge solid territory moyo that covers about an 8th of the board for the cost of only 6-7 stones.

Chew Terr · **#17**

Toge: As white, I tend to just take O17 and try to match moyo for moyo. I feel that the bottom right isn't urgent yet, because white still has options on either side that can be considred close enough to miai. That said, it's still iffy of whether or not white can build a moyo as efficiently as black, given black's 'head start'. I'd be eager to hear more opinions though, as you're higher-ranked than I am and I'm sure you knew everything I'm saying before you posed the question.

Bill Spight · **#18**

Numsgil wrote:

So I spent the weekend re-reading Wilcox's sector fights, where he spends a lot of effort talking about walls. I think I can make this a bit more productive of a conversation by helping to define some terms a little better. I'm going to present this information as authoritative, but it really only represents my novice understanding

Anyway, there are three kinds of plays that might be thought of as extensions:
1. Extension to form eye space (AKA safety extension)

Quote:

2. Extensions to form a moyo

Quote:

3. Extensions which attack

Quote:

4. Useless wall

Numsgil · **#19**

Bill Spight wrote:

1. Extension to form eye space (AKA safety extension)

Can you expand on this? I've seen this idea in a joseki before, too, where a variation has the extension made a bit closer to ward of future forcing moves against the group. What specific threat makes the move locally sente (I assume separating the wall and its extension)? And more importantly, what about moving closer to the wall prevents this from being sente?

Quote:

2. Extensions to form a moyo

What does "more than 5 spaces" mean? More than 5 spaces from what? If it refers to a rule or proverb it's not one I've ever heard, so I'm interested.

Quote:

4. Useless wall

Yes, if the white stones had to play low to approach black's thickness after it was built, black's thickness has at least partially payed for itself by passively limiting white's growth. Every approach would be gote and not super big.

As far as invading, I look at this and it might as well be a solid line of white stones. There are some maybe 3-4 point gote plays I see, but that will tend to wait until the late end-game before being made. Maybe there's a sequence I'm not familiar with?

Bill Spight · **#20**

Numsgil wrote:

Bill Spight wrote:

1. Extension to form eye space (AKA safety extension)

Can you expand on this?

Black's sagari makes this better for Black than the joseki, but I think that :w3:

is still a good play. As always, no guarantee of correctness.

Quote:

What does "more than 5 spaces" mean? More than 5 spaces from what? If it refers to a rule or proverb it's not one I've ever heard, so I'm interested.

If you leave 6 spaces between your stones, then your opponent has room to invade and then, if not pincered, to make a 2 space extension. Playing that far away from your existing stone or stones may be a large play, but we do not call it an extension.

Quote:

4. Useless wall

As far as invading, I look at this and it might as well be a solid line of white stones. There are some maybe 3-4 point gote plays I see, but that will tend to wait until the late end-game before being made. Maybe there's a sequence I'm not familiar with?

I now think that :b1:

is good. The position is rather complicated, and I have shown several variations. Some of the errors are intentional.

But probably not all of them.

Anyway, perhaps such positions won't look so solid anymore.

Walls, extensions, and frameworks

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