three sealed boxes in a row, joined by boundaries with little 0 handoff points between them. box A is linked to the first handoff, box C to the last. A and C are far apart and never meet. but if every boundary is a good DOOR, you can pass the link down the line — measure each middle 0 and the link hops across until A and C are joined, straight through the middle box, without A and C ever touching. it's a bucket brigade, but for entanglement. the catch: one broken boundary (a DRAIN) anywhere in the line and the bucket spills — the link dies at the break. a chain of doors carries it; one drain cuts it.