Life In 19x19

I challenged myself to make a Go variant without seki, and this is what I came up with. It differs from the regular game in the following aspects:

• There are two types of pieces: stones and ghosts. Each player uses stones and ghosts of their own color.
• A group is a maximal set of orthogonally contiguous pieces of the same color and type.
• A liberty of a group is a point orthogonally adjacent to it, provided that said point is either empty or occupied by a like-colored piece of a type different from the group. For example, a white ghost is a liberty of any orthogonally adjacent group of white stones, and a white stone is a liberty of any orthogonally adjacent group of white ghosts.
• On their turn, a player must perform exactly one of the following actions:
  • Pass.
  • Place a ghost of their color on an empty point.
  • Place a stone of their color on an empty point or on a point occupied by a ghost of any color. In the latter case, said ghost is then removed from the board.
  After a placement of any type, all enemy groups of any type without liberties are removed. After that, every friendly group of any type must have at least one liberty. Otherwise, the whole move is illegal and the player must choose a different one.
• Area scoring is used. On-board ghosts are added to the scores of their owners.
• For the purposes of ko and superko rules, two positions are considered identical if both have pieces of the same colors on the same intersections, even if piece types don't match.

I think ghost placement will only be desirable on seki intersections. Other than that, games played under these rules should look much like normal games.

Note that, without seki, fractional komi isn't needed, as there will be no ties.

Example

Click Here To Show Diagram Code

[go]$$B Not seki
$$ --------------------
$$ | a O b X c O d O X .
$$ | O O O X O O O O X .
$$ | X X X O O X X X X .
$$ | . . X X X X . . . ,
$$ | . . . . . . . . . .[/go]

Whoever moves first here can capture the enemy stones. If Black moves first, they can capture by placing a ghost at either b or c and, on their next turn, a stone at a or d, respectively. If White moves first, they can capture by placing a ghost at either b or c and, on their next turn, a stone at c or b, respectively.

Are ghosts only captured by placing a stone on them, or can they be captured by surrounding, like a normal stone? If the first, it seems like they'll completely change life and death.

Also are you sure they can't change semeai, even without a seki in play? A ghost stone lets you remove a shared liberty from your opponent without losing it for yourself.

hyperpape wrote:Are ghosts only captured by placing a stone on them, or can they be captured by surrounding, like a normal stone? If the first, it seems like they'll completely change life and death.

Groups of ghosts can be captured by surrounding, like normal groups.

hyperpape wrote:Also are you sure they can't change semeai, even without a seki in play? A ghost stone lets you remove a shared liberty from your opponent without losing it for yourself.

Hmm, you're right. Here's a simple capturing race taken from Sensei's Library where ghosts make a difference:

Click Here To Show Diagram Code

[go]$$B Approach moves make a difference
$$ ------------------
$$ |a O . 2 . 4 . . .
$$ |1 O O X X X O . .
$$ |3 X O X O O . O .
$$ |. X X O O . . . .
$$ |. . X O . . . . .
$$ |. . X O . . . . .[/go]

With normal rules, Black cannot win the semeai. Black must play before they can play atari at a, and by that time they are in atari themselves.

In my variant, however, Black can win by answering with a ghost placement at a, which puts only White in atari. If White answers with a ghost placement at a instead, Black simply gets for free and remains ahead in the liberty count (3 vs 2).

It seems this variant is less academic than I initially thought...