# Pass fight

Path: <= Fight
Difficulty: Intermediate   Keywords: Rules

Some variants of go rules lead to situations where one player may be able to gain from not passing at certain times near the end of the game, and a pass fight, similar to a ko fight, may develop.

For instance, suppose that the players are playing by a variant of the AGA rules using pass stones where the second pass in succession ends play without the requirement that White make the last play. Then we might see the following pass fight:

```Black: Pass.
White: Threat.
White: Pass.
Black: Pass.
```

White has gained a point by comparison to simply passing after Black's first pass because Black has had to give up two pass stones to White's one.

In the late 1980s, Terry Benson discovered pass-fights and avoided them in AGA Rules. Later and independently, pass fights under some simple German rules? with Territory Scoring were discovered by Bernd Gramlich?. Then he and RobertJasiek discussed pass fights under the Japanese 1997 Rules, but I do not recall which of us two mentioned pass fights there first. For no pass go pass fights were known earlier.[1] For other rulesets one should study history more carefully before one can be sure who may have discovered what. In particular, the "archives" of the mailing-list go-rules need to be studied.

RobertJasiek: I cite essential extracts of my old contributions to rec.games.go on 2001-01-04

## 1 - No-pass rules

Black to play

```s  := self-atari
t  := play on the board
(i.e. a threat avoiding illegally  passing)
{} := arbitrary, fixed number of
```
Black loses
Black wins

In general if the game starting from the empty position is

```# [{t}]; |{t}| even, then O wins
# [{t}]; |{t}| odd,  then # wins
```

Here in some sense the quality of pass fight play increases with |{t}|. However, the truth is that the entire game tree has two sorts of leaves: those at even and those at odd depth. Even depth means a O win, odd depth means a # win. Surely, mostly there are big subtrees with leaves of only one parity. E.g. in the example after an s such a big subtree follows. (Not necessarily after any s in all examples but in this example.)

Bill: Comment on the play: White's two point eye has a value (if independent) of -1.5. That gives the whole board a value of +0.5. Black wins by playing in White's eye at the cost of 0.5 move, yielding an eye worth -2 and a whole board worth 0, a second player win (for Black in this case). Filling his own eye costs Black 1 move, yielding a whole board worth -0.5, which is a White win.

## 2 - AGA rules (without White passing last, territory scoring)

Black to move; no prior pass; no prior prisoner

Perfect play

= = = pass.

Score: -2 (White wins by 2)

If is at , eliminating White's threat, the score is also -2. If = pass, the score is -1.

Here in general a game looks like

```# [{t}{ptt}pp]
```

and the core of the pass fight for player P is

```P [{ptt}pp]
```

If |{ptt}| is even, then P wins the pass fight about 1 extra pass stone. If |{ptt}| is odd, then P loses the pass fight about 1 extra pass stone.

{t} of the game is already decisive for who is going to win the pass fight. So one might even say that the pass fight starts with the game.

## 3 - Tromp-Taylor rules

White to move

at . = = pass.

Result: Jigo.

White loses

Score: +26

Correct play.

= = pass.

Score; -28

In general the pass fight about not being forced to play a p (pass) instead of playing an s because all other empty intersections would be S (suicide, the standard form of prohibited repetitive play) or K (all other prohibited repetitive play like in "Ing-hot" kos). A pass fight is [{ttk}pp] if won by the last player to play k or [{ttk}..pp] if won by the last player to play k if he also still plays within "..". Such a pass fight differs from "regular" "ko" fights, which are always [{ttk}..pp]. E.g. by winning a last endgame ko the winning player still plays in ".." by filling the ko mouth.

## 4 - Tromp-Taylor rules (using positional superko)

molasses ko; top right corner

Molasses ko

has just captured the ko.

```k := proper play in the ko
```

The pass fight is [{tkkkk}].

If the molasses ko is decisive for the game, then the pass fight is about gaining the right parity in the game tree outside the molasses ko while also respecting the side condition of (not) disturbing the nature of the cycle due to global features of superko.

## 5 - John Tromp's asymmetrical positions

John Tromp's asymmetrical positions (which I would have to dig up) have cycles of the types

```[{tkkkk}] or
[{tkkkkkk}]
```

I am not sure now whether pass fights with them are possible, too. Does anyone know?

So all pass fights above have one feature in common. They can be written to include {L}, where the number of moves in L is odd and where a) L includes plays on temporarily prohibited intersections or passes or b) {L} is followed by prohibited intersections only. [...]

I call everything of the following a restriction: no suicide rule, no repetition rule(s), game end rules. Therefore I should have called the defined object in my pass fights definition "restriction fights" instead. Then restriction fights are pass fights or ko fights. Ko fights are those fights in the middle of a game and pass fights are those still active just before the stop of a game. The nature of all restriction fights is similar: Both players fight about a restriction by using other plays as "threats" (moves suitable to lift or postpone a restriction for intersection(s)). Note: ko fights can be part of a pass fight, e.g. in no pass rules.

[1] Bill: Since neither player may pass in no pass go, how can it be a pass fight?

RobertJasiek: You can model no pass go by allowing passes but using the rule that the first player to pass loses the game. - Thx for your editing!

Bill: You're welcome. I thought it would look better. :-)

Path: <= Fight
Pass fight last edited by 192.75.48.150 on April 16, 2014 - 17:35