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🗺️ Local is not global

Every crossing can pass and the whole map can still jam. Check the crossings, predict, then try to refold it.

What the checker proves

For each map the engine searches every order of whole-line folds, exactly. A 2 × 4 map that passes at every crossing can still have no such order — the smallest case we found. Whether a fast rule decides flat folding for every size of map, with folds of any kind, has been open since Arkin, Bender, Demaine and colleagues posed it in 2004.