Fourfold · Echo Stones
Based on the USP 1.0 reference engine by OpenShobu, Apache-2.0.
Source: https://github.com/OpenShobu/Universal-Shobu-Protocol-Specifications/blob/4178b90af66d3834b465b6e9491f1f6c261c87ab/examples/javascript/engine.mjs
Modifications: Node imports and terminal transport removed; browser interface, heuristic opponent, practice positions, saves added.
Independent implementation; Shōbu is credited to Manolis Vranas and Jamie Sajdak. No publisher affiliation.
Rules profile: a blocked side loses; no automatic repetition draw.
Readable application source is included in engine.js and game.js.