John Fairbairn wrote:Bill
If I may be a terrier chasing the quarry first flushed out by Ed...
1. You say the problems with double sente has been known for at least 40 years. I infer from your quotation of Kano's 1974 book that this is the source for "40 years".
Yes. I also expect that Davies was aware of the divide by zero problem before 1974. Smart man.
You add that his text is confusing.
1a. Two points to that. First, I didn't find his text confusing. He is not teaching anything in that section. He is just defining terms, and he gives an example of a "pure" double sente before the one you cite.
Here is the diagram.
$$B "Double" sente
$$ --------------------
$$ | . . . . . . . O X .
$$ | . . . X O . . O X .
$$ | . . . X O . O O X .
$$ | . . . X X O . O X ,
$$ | X X X X O O O O X .
$$ | O O O O X X X X X .
$$ | . . . O O O . . . .
$$ | . . . . . . . . . .
- Click Here To Show Diagram Code
[go]$$B "Double" sente
$$ --------------------
$$ | . . . . . . . O X .
$$ | . . . X O . . O X .
$$ | . . . X O . O O X .
$$ | . . . X X O . O X ,
$$ | X X X X O O O O X .
$$ | O O O O X X X X X .
$$ | . . . O O O . . . .
$$ | . . . . . . . . . .[/go]
By absolute counting a play has a value of 20, with follow-ups worth 17 points for Black and 19 points for White. In theory this is a gote, but in practice it will almost certainly be a double sente, because plays worth 17 points or more are rare in go. The opening move gains only about 14 points, for instance.
The one you cite he presents to make a clear point: "However, although this is likewise called double sente you need to recognise that here a difference may arise in the right [to play it] depending on the position." He then goes on to explain that if Black has a higher priority move elsewhere, he may take that instead of fully answering in the cited position in gote. Assuming that's the portion you found confusing (and there's little else in the source), I'm puzzled because it seems straightforward to me - it's double sente but in practice White is more likely to get it.
I consider it confusing because it is Black sente, but in practice sometimes White can play with sente, too. Kano gets it backwards.
Also, it may be worth adding that he does not use the phrase unconditional double sente, though I accept that some such sort of qualification can be inferred.
Back in the 1980s I submitted an article to
Go World entitled "There is No Such Thing as Double Sente". I had written it some years earlier, but had recently reestablished contact with Bozulich, so I thought I would give it a try. As of 2000 I had come to see that there is a well defined sense of double sente which depends upon the whole board. IOW, it is conditional. That's why I used the term, unconditional, for the traditional usage of double sente.
1b. However, I may have missed something, and in support of that, the 1985 edition of Kano's dictionary reworks this section entirely, which perhaps indicates that he saw a problem with his first effort. But the problem may not have been in the position cited. Rather it may be because he appears to have taken a completely different approach to boundary play counting in general, and to my inexpert eye he has moved a long way towards O Meien, even using some of the same "rights" vocabulary. In the 1985 book Kano also goes well beyond his first effort in explaining why we need to be cautious about regarding the components of a double sente position as sente, and in support of that he cites a completely different kind of position where life & death are not involved, but aji is, and he adds a section on ajikeshi. However, he also clearly explains that this is really a middle game issue and not a boundary play issue and adds that "In a sense it can be called a pre-boundary play problem, but if we do classify it as a boundary play, it is one to which we will give the highest order of priority." And, also worth adding, he tends to be scrupulous in saying "double sente boundary play" and not just "double sente." Again, I see no confusion in this. So my question boils down to this: why should I care about things like dividing by zero, why should I believe there is any practical value in accepting there may (or may not) be some pedantic flaw in the concept of "double sente", especially when I do not add "unconditional" to the term and I do recognise the difference between middle game and endgame?
IMO, the value is clarity of thought. O Meien does not need the theoretical concept of double sente. Neither does CGT. (I think that the proverb about double sente is valuable, but needs interpretation. In practice, such positions do arise.) I remember, as a 4 kyu, seeing a "double sente", a second line kosumi, go unanswered in a pro game. Tilt! Had I been playing a teaching game with, say, a 7 kyu who ignored that play, I would have said, "No, that's a double sente. You have to answer." Wrong! If pros ever believed that that play was double sente, they overcame that notion. Better not to learn it in the first place.
I do not think that Kano was the only one who was reevaluating the idea of double sente at that time. That it was problematical was already in the air.
2. As I just mentioned, Kano appears to have moved towards the O Meien position even before O did. But O took it further and presents a completely different system - Absolute Counting. I'm in no position to judge on the relative merits of any system, but I think I can judge when one system is starkly different from another. On that basis, I fail to see how lack of mention of double sente in O's system is relevant. He doesn't see the need to use it, so he doesn't seem to regard it as relevant either. He doesn't even bother dismissing it. It's like a paper spoon. He just doesn't need it in his system, so why mention it. He's not saying it's not relevant to other systems of boundary play counting. Instead, he is saying his whole system is better than other systems, but he doesn't appear to have convinced the whole go world yet. Is that a fair summary?
I have touched on this above. O knows what he is doing. He even evaluates reverse sente but not sente, which is a clarification.
I don't know about his influence on others, but by demonstrating that you do not need the concept of double sente for evaluation, I think that others will come around to that way of thinking, sooner or later.
I have the 1995 version (3d printing in 2000) of the Nihon Kiin's
Small Yose Dictionary. Unfortunately, it starts out with double sente, and even follows up with a whole board position with a number of "double sente". It shows two lines of play, one with Black playing first and one with White playing first. In each line of play the first player plays
all of the double sente with sente. Tilt! Sakata would never have answered each "double sente", nor would Go Seigen, nor would Fujisawa Hideyuki, nor would Cho Chikun, nor would Gu Li, nor would Lee Sedol, nor, I trust, would any Chinese, Korean, or Japanese pro at the time. That is a prime example that shows the value of clear thinking about so-called double sente.
So far, so bad. But! There is a section showing a number of plays and evaluations. Unlike such sections in earlier editions, no double sente are evaluated. Why not? Surely because it was known that there is something wrong with evaluating double sente.
is not good. White can even play this if there's enough ko threats so black is 1 1/3 points worse off that the above diagram. 
