Meigo: a naturally finite game that plays almost like Go

[luigi]

Meigo game won by Black. The moves before the first marked stone placement are omitted. White resigned at the end.
Meigo is a Go variant that makes infinite loops impossible without using superko rules. The basic idea is that placements in enemy territory are made with marked stones, and you cannot place a marked stone and remove a marked stone on the same turn if the last stone placed on the board was marked. The result is a game that plays almost like Go, as you can see in these sample games:

• A 9x9 game.
• The 13x13 game displayed above.
• A 19x19 game following the moves of the longest known high-level Go game, which spans 417 moves (twice as long as a typical Go game) and features a whopping 79 ko captures. Recreating the game under Meigo rules requires making 4 illegal ko captures: moves 82, 304, 409 and 415. The first two of those are preceded by enemy ko captures, so making them illegal avoids potential n-tuple-ko cycles.

I recommend replaying the games with BesoGo. It's nicer than EidoGo, but has no hosting service.

[RobertJasiek]

placements in enemy territory are made with marked stones, and you cannot place a marked stone and remove a marked stone on the same turn if the last stone placed on the board was marked.

"Territory" is a term with a different meaning. You mean "empty 'region' (or call it 'string') only surrounded by (that is, adjacent to) one player's stones".

You mean "may not" instead of "cannot".

From your text, marks are permanent (at least) as long as stones remain on the board. Is this your intention?

[Javaness2]

Does anyone play there variants which you keep inventing?

[luigi]

placements in enemy territory are made with marked stones, and you cannot place a marked stone and remove a marked stone on the same turn if the last stone placed on the board was marked.

"Territory" is a term with a different meaning. You mean "empty 'region' (or call it 'string') only surrounded by (that is, adjacent to) one player's stones".

Right. That's the way I usually define territory in my games, though.

You mean "may not" instead of "cannot".

You may or may not be right on that one.

From your text, marks are permanent (at least) as long as stones remain on the board. Is this your intention?

Right, you unmark a stone by merging it with an unmarked stone of your color, and you need to remove an enemy group surrounding your marked stone in order to merge it with an unmarked stone of your color.

[luigi]

Does anyone play there variants which you keep inventing?

Hopefully someone will. This one is pretty good.

[EricBackus]

Right, you unmark a stone by merging it with an unmarked stone of your color, and you need to remove an enemy group surrounding your marked stone in order to merge it with an unmarked stone of your color.

• What if a string of one color subsequently gets surrounded by the other color? Do the stones in the inner string become marked? Are subsequent "inner color" plays within this same region marked? It seems like for a given board position, some stones may be marked or not marked depending on the order of previous moves.
• What if a string of one color surrounds a string of another color, and then the inner string ends up surrounding enemy stones within itself? Are the inner-most enemy stones marked? I can easily imagine this affecting life-and-death.

[luigi]

Right, you unmark a stone by merging it with an unmarked stone of your color, and you need to remove an enemy group surrounding your marked stone in order to merge it with an unmarked stone of your color.

• What if a string of one color subsequently gets surrounded by the other color? Do the stones in the inner string become marked? Are subsequent "inner color" plays within this same region marked? It seems like for a given board position, some stones may be marked or not marked depending on the order of previous moves.
• What if a string of one color surrounds a string of another color, and then the inner string ends up surrounding enemy stones within itself? Are the inner-most enemy stones marked? I can easily imagine this affecting life-and-death.

That's not how it works. Marked stones can be unmarked, but a stone that was unmarked when placed never becomes marked. For example, in this game, does not mark the black stone at A8.

The rules say:

To play, follow these steps:

1. Select an empty point.
2. If the selected point is orthogonally adjacent to both marked and unmarked friendly stones, unmark (by flipping them) all marked friendly stones reached by the selected point.
3. If the selected point reaches a marked friendly stone or no friendly stones at all, place a marked friendly stone on that point. Otherwise, place an unmarked friendly stone on that point.
4. Move to the prison all enemy chains without liberties. After this, the stone you just placed must be part of a chain with at least one liberty.

A point reaches a stone if a path along the lines of the board can take you from one to the other without stepping onto an unmarked stone. Note that the path can go through marked stones of any colors.