Let's consider this position

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[go]$$B
$$ ------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . O . .
$$ | . . . O . O . . . , . .
$$ | . . O . . . . . . . . .
$$ | . . X X . . . . . . . .
$$ | . . . . . . . . . . . .[/go]

Pros are entering at a and b, but never at x. Amateurs do and the bots consider this a valid option too.

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[go]$$B
$$ ------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . x . . . . . . .
$$ | . . a . . . . b . O . .
$$ | . . . O . O . . . , . .
$$ | . . O . . . . . . . . .
$$ | . . X X . . . . . . . .
$$ | . . . . . . . . . . . .[/go]

Let us explore a possible line in this game:

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[go]$$B
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . O . . . . X . . . . |
$$ | . . . O . O . . . , . . . . . , X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Kudo Norio, 9d - Ishida Yoshio, 9d, B+3.5 (Komi 5.5)
34th Honinbo League, 1979-02-15

Looks playable to me too. What do you think; Why the invasion at x in the top diagramm is never touched in professional games?

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[go]$$B
$$ ---------------------------------------
$$ | . . . . . . . . . B . . . . . . . . . |
$$ | . . . . 1 . . . . 9 0 . . . . . . . . |
$$ | . . 2 . . . 5 3 7 O . . . . X . . . . |
$$ | . . . O . O 6 4 . 8 . . . . . , X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

And perhaps we could learn from the bots in hindsight, the more sensible extension for white is perhaps:

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[go]$$B
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . W . . . . . X . . . . |
$$ | . . . O . O . . . , . . . . . , X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

But I am also very interested in your views, if you beg to differ. (The pros probably know quite well what they are doing )

Cultural blind spot.

It's not like pros have not identified x as a weak point. However, in the history of go the position in the top left has scarcely occurred before the 20th century, if at all. And the invasions at a and b have a much longer history in similar shapes. I suspect that they produced satisfactory results in the estimation of humans, and no further exploration was done.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

That extension appears 20 times in Waltheri. I recall that Segoe recommended it in some positions in his Strategy Dictionary (Sakusen Jiten).

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Kudo Norio, 9d - Ishida Yoshio, 9d, 1979-02-15

Click Here To Show Diagram Code

[go]$$Bcm33 Elf's mainline variation for
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . 1 . 5 . . . . . . . . . . . . |
$$ | . . 3 . . . 2 . . O . . . . X . . . . |
$$ | . . . O . O . . . , . . . . . , X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . 4 X X . . . . . . . . . . . . . . . |
$$ | . . 6 . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . a . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Ishida did not invade, but played the checking extension at a on the right side. In its commentary Elf prefers the invasion by 7½%. I suspect that this variation is psychologically difficult for humans, as each side surrenders what it has staked out, and the evaluation is not easy.

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[go]$$Bcm33 Elf's variation 2
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 4 1 3 . . . . . . . . . . . . . |
$$ | . . . . 2 . . . . O . . . . X . . . . |
$$ | . . . O . O 5 b . , . . . . . , X . . |
$$ | . . O . . . a . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

This is Elf's second choice for , with a winrate estimate 1½% less. Elf stops at because of insufficient playouts for later plays. I expect that the variation will continue Black a - White b.

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[go]$$Bcm33 Elf's variation 3
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . 1 . . . . 7 8 . . . . . . . . |
$$ | . . . 2 . . . 3 5 O . . . . X . . . . |
$$ | . . . O . O . 4 6 , . . . . . , X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

is Elf's third choice, with a winrate estimate 2½% less. OC, both estimates are within Elf's margin of error.

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[go]$$Bcm41 Elf's variation 3, continued
$$ ---------------------------------------
$$ | . . . . . . . . . . a . . . . . . . . |
$$ | . . . . X . . . . X O 3 . . . . . . . |
$$ | . . . O . . 4 X X O 1 . . . X . . . . |
$$ | . . . O . O . O O 2 . . . . . , X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . X O . . . . . . . . . . . . . . . . |
$$ | . X O X O . . . . . . . . . . . O . . |
$$ | . . X O O . . . . . . . . . . . . . . |
$$ | . . X , . . . . . , . . . X . X . . . |
$$ | . . X O O X . . . X . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

This is as far as this Elf variation goes. Black may continue at a, or maybe tenuki.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Bill, thanks for your deeper look. Did you check Elf's opinion on 34 at C17 as well?

Not on Elf's radar. IOW, it got no more than 1500 playouts. Elf's third choice got 3.9k playouts. The Elf team plainly does not trust plays with fewer than 1500 playouts. I don't know about 1500 exactly.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Elfv1 is not fond of the sequence.

Err, which sequence? Thanks.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

This one