I discovered the Trike meta-rule or mutator, which can be used with any partisan, pure placement game on hexagonal or square grids, such as Hex, Y, Havannah, Majorities, Atoll, and many others. The Trike stone-placement protocol limits the set of empty
cells available to players, to only the moves of a neutral pawn.

Trike meta-rule:
Materials: use the appropriate board, and a neutral pawn, black/white checkers, stones, or two differently colored pens.
Players take turns moving a neutral pawn around on the board (passing is not allowed).
The neutral pawn can move any number of empty points, in any direction in a straight line, but cannot move onto, or jump over occupied points.
When a player moves the pawn, first they place a checker of their own color, onto the destination point. Then they move the pawn on top of it.
It follows that the pawn will inevitably get trapped, and the players must always choose which version is being played, from three options.
Trike Zero: When the pawn is trapped, the game is over.
Trike-Even: players finding themselves with the pawn trapped can place anywhere.
Trike-Odd: players who traps the pawn can place anywhere.

Meta-rules are not games on their own, but hundreds of new games can be invented using meta-rules, and Trike is no exception. Here is one example that I have come up with.

Pure Trike: This is probably the simplest game that naturally emerges from the Trike rule.
Play on an equilateral triangular hexagon-tessellated grid (like the one used for Y) with sides of length 9.
Use Trike meta-rule (Zero).
At the end of the game, each player gets a point for each checker of their own color adjacent to, or underneath, the pawn.
The person with the highest score wins.
Implement pie rule for serious play.

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