Incursion Go is a Go variant where standard ko rules are not used. Instead, it is illegal to make an incursion if your opponent's last move was an incursion.

An incursion is a placement in an enemy territory. A territory is an empty point along with all points one can reach from it through a series of steps onto orthogonally adjacent empty points. An enemy territory is a territory such that all stones orthogonally adjacent to points in the territory are of your opponent's color.

All other rules are the same as in Go. Suicide of one or more stones is not allowed, and area scoring is used.

All known forced Go cycles are impossible in this variant. Unlike the related variant Stoical Go, Incursion Go preserves snapbacks.

If you place a stone on your turn and, by the end of it, the board position is the same as the board position at the end of any of your previous turns, the game ends in a draw. However, if forced cycles are impossible, this will require cooperation between the players, which should not occur in actual play. However, if forced cycles are impossible, this will require cooperation between the players, which should not occur in actual play.

In a more radical, less restrictive implementation of this idea, an incursion is said to be minimal if it captures exactly zero or one groups, and it is illegal to make a minimal incursion if your opponent's last move was a minimal incursion. Non-minimal incursions are always legal. This also prevents all known forced cycles.

Luis Bolaños Mures designed Incursion Go in April 2023.

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[go]$$W Eternal life, a Go cycle, is analogous to a dead ko in Incursion Go: Since is an incursion, Black cannot reply with another incursion at the vital point a, but must make a ko threat first, which must itself not be an incursion. Once Black runs out of ko threats, the black group will die.
$$ -------------------------------------------------
$$ - . O O X . - . O O X . - . O O X . - . O O X . -
$$ - X X O X . - X X O X . - X X O X . - X X O X . -
$$ - . X O X . - 1 X O X . - . X O X . - . X O X . -
$$ - O X O X . - O X O X . - . X O X . - 3 X O X . -
$$ - . O O X . - . O O X . - 2 O O X . - . O O X . -
$$ - X O X . . - X O X . . - X O X . . - a O X . . -
$$ - O X . X . - O X . X . - O X . X . - O X . X . -
$$ - . X . . . - . X . . . - . X . . . - . X . . . -
$$ - . . . . . - . . . . . - . . . . . - . . . . . -
$$ -------------------------------------------------[/go]

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[go]$$B Moonshine Death: In Go, the white group in the top left corner is alive because White has an unlimited number of unremovable ko threats in the top right corner. In Incursion Go, both corners are proper dead kos for White because and those ko threats are all incursions.
$$ ---------------------------------------
$$ | O 1 O X . . . . . . . . . O X . X O . |
$$ | . O X X . . . . . . . . . O X X O O O |
$$ | O O X . . . . . . . . . . O X . X O . |
$$ | X X X , . . . . . , . . . O X X X X O |
$$ | . . . . . . . . . . . . . O O O O X X |
$$ | . . . . . . . . . . . . . . . . O O O |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]

So, you can't kill a group with nakade immediately after your opponent killed a group with nakade?

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A good system naturally covers all corner cases without further effort.

Yes, but not only that. A nakade is an incursion, but so is a throw-in, for instance. Also, the rule affects capturing and non-capturing incursions alike.

By the way, since all incursions in all known forced cycles are made in line-shaped territories, with no loops or forks, the rule can easily be made less restrictive by defining an incursion as a placement in a line-shaped enemy territory. This way, placements in enemy territories that are not line-shaped are always legal.

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[go]$$B
$$ -----------------
$$ | . X O O O X . . -
$$ | X O 1 . O X . . -
$$ | O O O O O X . . -
$$ | X X X X X X . . . -
$$ | . . . . . . . . . -
$$[/go]

Is it a good understanding to say that above kills the white group?

Yes, in the standard variant, if White doesn't have a big enough ko threat, the white group dies. Nice example!

However, in the variant with minimal incursions, White is alive because a white play in the corner captures two groups and is thus not a minimal incursion.

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[go]$$B
$$ -----------------
$$ | . X O O O O X . . -
$$ | X X O 1 . O X . . -
$$ | O O O O O O X . . -
$$ | X X X X X X X . . -
$$ | . . . . . . . . . -
$$[/go]

same idea with minimal incursion. Without ko threat white dies doesn't it?

Oops, seeing a loop is possible with cooperation of the players what is the real interest of this variant comparing to AGA rule with superko in order to avoid any loop?

That's correct!

The idea is to prevent forced cycles without forcing players to keep track of all previous positions. Cooperative cycles are not a problem, in my opinion.

More ideas:

Excursion Go 1

On your turn, perform the following actions in order:

1. Select an empty point.
   
2. If the point is in enemy territory or next to a friendly king, place a friendly king on the selected point and replace all adjacent groups of friendly stones with groups of friendly kings. Otherwise, place a friendly stone on that point and, for each of the players' colors, replace every king of that color on the board with a stone of that color.
   
3. Remove all enemy groups without liberties.
   
4. Remove all friendly groups without liberties.

You cannot place a king and remove a king on the same turn.

Excursion Go 2

On your turn, perform the following actions in order:

1. Select an empty point.
   
2. If the point is in enemy territory or next to a friendly king, place a friendly king on the selected point and replace all adjacent groups of friendly stones with groups of friendly kings. Otherwise, place a friendly stone on that point.
   
3. Remove all enemy groups without liberties.
   
4. Remove all friendly groups without liberties.

You cannot place a king and remove a king on the same turn if your opponent just placed a king.
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Both variants should change Go's life and death less than Incursion Go. I wonder if the second one is hard-finite (meaning cycles are impossible even if both players collude to create one). If it is, modifying Square Lifeline in the same fashion should also produce an interesting hard-finite variant.

Under these rules, White lives in both of Gérard's examples.

What do you mean by ”king” instead of ”stone” ?

In those variants, there is a second type of piece called "king".