prisoner return
In Mathematical Go Berlekamp and Wolfe introduce the idea of prisoner return. It is an additional rule to be applied to no pass go. With prisoner return, a player can pass but only if she has one or more prisoners. On passing, the player must return one of these prisoners to her opponent. This is similar to the use of pass stones in AGA rules. As usual, the first player not to have a valid move loses.
Since the original notion of a pass is to do nothing at all, we may still consider play with prisoner return as a form of no pass go. As such, it is close to territory scoring with a group tax, which is the oldest known form of go scoring.
The Count: It's like area scoring with a group tax, right?
Bill: Area scoring with group tax, aka stone counting, persisted in China into the 20th century. But the oldest known form of go scoring is territory scoring with a group tax! See Ancient Chinese Rules and Philosophy.
The Count: No, no, I didn't mean that. I meant, "Isn't no-pass go with prisoner return equivalent to area scoring with a group tax, not territory scoring with a group tax?"
Bill: Well, with some exceptions, when a position has a value that is a number, in CGT terms, that number equals the territory score with a group tax. At the same time, dame matter, since a dame has a value of * (Star). So a position with a score of 0 and a dame (*) is a first player win, not jigo. In that way it is like area scoring with a group tax.
The Count: Okay. I'm not too hot on the CGT stuff, but I think I understand the main point: it can be summarised as "The miai value using prisoner-return counting is the same as the one with territory scoring (i.e. 1 point less than with area scoring)". I'll add some stuff later about how the result is the same as for stone scoring with half a point (more) komi. Would you agree that what is there currently is a bit misleading though? Can you change it or give me the go-ahead to?
Bill: The main thing I was interested in saying was that no pass go with prisoner return is close to the oldest known form of go scoring. As you point out, it is also close to stone counting, and I think that is well worth saying, too. Please go ahead with an edit. :-)
The Count: Okay, and just to check – White must make the last pass in the ancient scoring, right? I wasn't sure whether this was meant, or just the alternation of moves.
Bill: I don't know about the last pass in ancient scoring. That's what Hu claims, right? But I don't think that John Fairbairn mentions it.